Fixes #15849

The v0.12 language supports numeric constants only in decimal notation, as
a simplification. For rare situations where a different base is more
appropriate, such as unix-style file modes, we've found it better for
providers to accept a string containing a representation in the
appropriate base, since that way the interpretation can be validated and
it will be displayed in the same way in the rendered plan diff, in
outputs, etc.

We use tv.Value() here to mimick how HCL 1 itself would have interpreted
these, and then format them back out in the canonical form, which
implicitly converts any non-decimal constants to decimal on the way
through.

The re-introduction of some of the ambiguity between argument and nested
block syntax (for compatibility with existing provider patterns)
unfortunately leads to some interesting consequences for attributes using
this mode.

While the behavior is generally as before in straightforward cases, this
page aims to spell out some of the different usage patterns explicitly
for the benefit of those writing more complex configurations, such as
generic re-usable modules where using argument vs. block syntax leads to
some real differences.

This page is intentionally not linked from anywhere in the part of the
website maintained in the Terraform repository. Instead, it can be linked
from the provider documentation for any argument where this pattern is
used, to help users understand the ways in which that argument might
deviate from the usual behaviors of arguments vs. nested blocks.

* fix docs typo

* restore trailing newline

Previously it was calling directly to hcldec.Variables, and thus missing
the special fixups we do inside ReferencesInBlock to deal with
Terraform-specific concerns such as our attribute-as-blocks preprocessing.

Allow all planned computed diff fields

Stripping these was a patch for some provider behavior which was fixed
in other ways, and is no longer needed.
Removing this allows us to implement correct CusomizeDiffFuncs in
providers so that they can mark fields with empty values as computed
during a plan.

These are the largest source of the old "diffs didn't match after apply"
errors. It's almost always an upstream dependency that caused the final
error.

Fixes #20917

Validate configuration shims for nulls in lists

* vendor: update hcl2 dependency
* command/format: revert diagnostic format behavior if snippet or highlight range is empty

This reverts commit 97bde546.

A list-like attribute containing null values will present a list to
helper/schema with nils, which can cause panics. Since null values were
not possible in configuration before HCL2 and not supported by the
legacy SDK, return an error to the user.

helper/schema: Prevent setSet() panic with typed nil

increase grpc recv limit

Some providers may generate quite large schemas, and the internal
default grpc response size limit is 4MB. 64MB should cover most any use
case, and if we get providers nearing that we may want to consider a
finer-grained API to fetch individual resource schemas.

filter nulls when shimming a config

Nulls can't exist in HCL, and the legacy sdk will panic when
encountering them. The map shim already did this, just copy the same
pattern in the list case.

References:

* https://github.com/hashicorp/terraform/issues/14418
* v0.9.5 (original bug report): https://github.com/hashicorp/terraform/blob/a59ee0b30e1350d18e44a3a2a405124302751f93/helper/schema/field_writer_map.go#L311
* v0.11.12 (Terraform AWS Provider discovery): https://github.com/hashicorp/terraform/blob/057286e5228559722bfc9bf6a03679650358c9d2/helper/schema/field_writer_map.go#L343

When creating flatten functions in Terraform Providers that return *schema.Set, its possible to return a typed `nil`, e.g.

```go
func flattenHeaders(h *cloudfront.Headers) *schema.Set {
	if h.Items != nil {
		return schema.NewSet(schema.HashString, flattenStringList(h.Items))
	}
	return nil
}
```

This previously could cause a panic, e.g.

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x1881911]

goroutine 1325 [running]:
github.com/hashicorp/terraform/helper/schema.(*MapFieldWriter).setSet(0xc00054bf00, 0xc00073efa0, 0x5, 0x5, 0x5828140, 0x0, 0xc0002cea50, 0xc000e996a8, 0xc001026e40)
	/Users/bflad/go/pkg/mod/github.com/hashicorp/terraform@v0.11.12/helper/schema/field_writer_map.go:343 +0x211
```

Here we catch the typed `nil` and return an empty list flatmap result instead. Unit testing result prior to code update:

```
--- FAIL: TestMapFieldWriter (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
  panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x1777cdc]

goroutine 913 [running]:
testing.tRunner.func1(0xc00045b800)
  /usr/local/Cellar/go/1.12.1/libexec/src/testing/testing.go:830 +0x392
panic(0x192cf20, 0x2267ca0)
  /usr/local/Cellar/go/1.12.1/libexec/src/runtime/panic.go:522 +0x1b5
github.com/hashicorp/terraform/helper/schema.(*MapFieldWriter).setSet(0xc0004648a0, 0xc0004408d0, 0x1, 0x1, 0x19e3de0, 0x0, 0xc00045c600, 0x30, 0x19e0080)
  /Users/bflad/src/github.com/hashicorp/terraform/helper/schema/field_writer_map.go:344 +0x68c
github.com/hashicorp/terraform/helper/schema.(*MapFieldWriter).set(0xc0004648a0, 0xc0004408d0, 0x1, 0x1, 0x19e3de0, 0x0, 0x1, 0x18)
  /Users/bflad/src/github.com/hashicorp/terraform/helper/schema/field_writer_map.go:107 +0x28b
github.com/hashicorp/terraform/helper/schema.(*MapFieldWriter).WriteField(0xc0004648a0, 0xc0004408d0, 0x1, 0x1, 0x19e3de0, 0x0, 0x0, 0x0)
  /Users/bflad/src/github.com/hashicorp/terraform/helper/schema/field_writer_map.go:89 +0x504
github.com/hashicorp/terraform/helper/schema.TestMapFieldWriter(0xc00045b800)
  /Users/bflad/src/github.com/hashicorp/terraform/helper/schema/field_writer_map_test.go:337 +0x2ddd
testing.tRunner(0xc00045b800, 0x1a44f90)
  /usr/local/Cellar/go/1.12.1/libexec/src/testing/testing.go:865 +0xc0
created by testing.(*T).Run
  /usr/local/Cellar/go/1.12.1/libexec/src/testing/testing.go:916 +0x35a
```

[Website] Update provider name

In order to preserve pre-v0.12 idiom for list-of-object attributes, we'll
prefer to use block syntax for them except for the special situation where
the user explicitly assigns an empty list, where attribute syntax is
required in order to allow existing provider logic to differentiate from
an implicit lack of blocks.

We don't appear to actually need to constrain these, and removing these decls may help avoid the need for `bzr` to install all the dependencies.

Better handling of unknowns in sdk shims

It turns out that collections containing only unknowns could be lost,
meaning there wasn't a direct correlation between the unknown and null
value which would have otherwise been restored.

NewRemoved diff values were somtimes left in maps and lists.

The legacy diff process inserts unknown values into an optional+computed
map. Fix these up in post-plan normalization process, by looking for
known strings that were changed to unknown.

Because schema.ResourceDiff can't differentiate between unknown
values and new computed values, unknowns can be lost during an update.
If a planned value converted an unknown to a null, restore the unknown
so that it can be correctly replaced in the final plan.

backend/remote: correctly load remote variables

* configs/configupgrade: detect invalid resource names and print a TODO
message

In terraform 0.11 and prior it was possible to start a resource name
with a number. This is no longer valid, as the resource name would would
be ambiguous with number values in HCL expressions.

Fixes #19919

* Update configs/configupgrade/test-fixtures/valid/invalid-resource-name/want/resource.tf
Co-Authored-By: mildwonkey <mildwonkey@users.noreply.github.com>