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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Thank you! However, we're getting pretty late in the release cycle and so I'd like to see this backed out of 19.x ASAP (and given the direction the discussion is going, I think it should be backed out of main as well).

@qiongsiwu will you be able to get these changes reverted today? If not, I'll try to revert them this afternoon. CC @tstellar @tru for release manager awareness.

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