[clang] [clang] Function type attribute to prevent CFI instrumentation (PR #135836)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I only have minor suggestions, else this looks good to me.  Please give Aaron a chance to take another  look as well.

https://github.com/llvm/llvm-project/pull/135836