[clang] [flang] Add IR Profile-Guided Optimization (IR PGO) support to the Flang compiler (PR #136098)

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

If I understand what you are saying correctly, this would involve, for example, duplicating the definition of `enum ProfileInstrKind` in `flang` in this PR, then a second PR that moves it from both `flang` and `clang` to `llvm/Frontend`. This is churn that can be distracting. The changes to `clang` in this case are unlikely to be major and getting it reviewed and approved should not be difficult. I would suggest doing those in this PR. 

https://github.com/llvm/llvm-project/pull/136098