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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I have not reviewed other pragmas that we do not support, but I think this one is a bit special in that it defines functions (code) that can be executed, and we are aware of use cases where detecting the error at link time is insufficient.

Ooh, maybe I misunderstood the situation. I didn't think this was something detected only at link time, I was thinking this added a symbol at compile time and so the user would get an "unknown identifier 'whatever'" diagnostic anyway if they tried to call the function. Is that not the case?

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