This was an attempt to try to report more context in an unknown for_each
error by dragging unknown value origin information through the normal
expression evaluation process using cty marks.

However, it doesn't actually work because Terraform evaluates each
expression separately in its own evaluation context, and so this trick
of sneakily re-evaluating an expression with marks only when we hit an
error can only ever find shallow references, which we can already find
statically anyway.

I'm just pushing this up to keep it for a possible second attempt later.
I think a second attempt would need something more like the analyzer
I built in #28944, but using marks and expression evaluation to deal with
its analyses instead of the custom static technique I tried there.