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

[Aaron Ballman via cfe-commits]

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

LGTM! Please make sure 1) this gets backported to the 19.x branch (https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches), 2) the 19.x release branch gets a release note for the pragma and command line options.

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