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

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

Sorry about the delay! I was on break. Thanks for the comments @AaronBallman and @cor3ntin ! 

> I think it would make sense to revert this and the original patch (so there's no #pragma mc_func support in Clang 19.x or main at all) so we can take a fresh run at a new PR to see all of the impacts and consider the design.

I have relayed the comments internally and we are discussing it. I will let you know as soon as possible. 

> Should we consider a feature like -Wunknown-pragmas=<pragma-name> rather than doing something specific for that one pragma?
> Have you consider whether this use case generalizes at all?

No we were not aware of other use cases. But `-Wunknown-pragmas=<pragma-name>` sounds like a good idea! Do you think this can be useful in other cases? How do we envision this warning/error to work with the existing `unknown-pragmas` warning if they are both present? I will draft a discourse RFC if this is sufficiently general/useful.  

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