[llvm] [UTC] Support to test annotated IR (PR #165419)

[Alexander Richardson via llvm-commits]

https://github.com/arichardson commented:

We also have a `--check-attributes` option to check another subset of comments. Could you add a test showing whether the new --check-comments flag also checks the function attributes comment?

https://github.com/llvm/llvm-project/pull/165419