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

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

>  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.

Yes I agree. This is at the moment off by default only because certain AIX system headers contain `mc_func`. If the user includes such headers in their program, they may see compile errors, but there is nothing they can do to their source code to fix the problem. We are working on removing all `mc_func` uses from the system headers. Once that is done, we will flip the default behaviour so that the error check is always on. That said, I don't find it unreasonable either to turn on this error by default earlier. The user can use the `no-err` flag to intentionally turn it off. Will bring this back internall for a quick discussion. 

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