[clang] [AIX] Detect `#pragma mc_func` (PR #99888)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Oh, this is even weirder than I realized -- that example gives a declaration for `add_logical()` (line 6), so that's why there's no "unknown function" diagnostic from the compiler, just a link error. That's a plausible reason for this to be handled in a special way, but now I'm wondering why this option isn't on by default for AIX. I don't think anyone is going to know to turn this option on, whereas if it's on by default, users have to make a decision as to how to proceed, which seems like the better default.

https://github.com/llvm/llvm-project/pull/99888