[clang] [AIX] Turn on `#pragma mc_func` check by default (PR #101336)

[Fangrui Song via cfe-commits]

MaskRay wrote:

(Tips: Personally when I have some patches intended for the release branch, I'll build and test llvm-project in the release branch as well to decrease the risk of potential immediate reverts/churn.)

https://github.com/llvm/llvm-project/pull/101336