[clang] Ofast deprecation clarifications (PR #101005)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

We have instructions at https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches but the basic gist of it is to add a special comment and add the PR to the correct milestone. As in:

/cherry-pick 48d4d4b641702bf6db03a1bac73b7e13dea28349

https://github.com/llvm/llvm-project/pull/101005