Summary: This updates to clang-9 and also updates our llvm build to use 9.0

Test Plan: `bazel test //...` and jenkins.

Reviewers: oazizi, #engineering

Reviewed By: oazizi, #engineering

Differential Revision: https://phab.corp.pixielabs.ai/D2235

GitOrigin-RevId: 1bfc34c0d46fadca0a1c3f53f2ac9a675e9795c4