[clang] [llvm] [InstrPGO] Instrument sampling profile based cold function (PR #109837)

[Ellis Hoag via cfe-commits]

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

The backend looks good to me! (after resolving my last comments of course)
Thanks for adding this feature!

https://github.com/llvm/llvm-project/pull/109837