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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman sorry to bug you. This patch was not tested sufficiently on my end and it is causing build failures on AIX buildbots due to the aforementioned system headers including `#pragma mc_func`. See https://lab.llvm.org/buildbot/#/builders/64/builds/582/steps/5/logs/stdio for example. The compiler-rt cmake files need some updates to add the `no-err` flag. Additionally, there are tests that needs to be updated (we need to augment the tests with `-fno-err-pragma-mc-func-aix` e.g. https://lab.llvm.org/buildbot/#/builders/64/builds/550). Do you think it is reasonable to revert this PR for now (and from llvm19)? We can reland the PR once these necessary changes are made in one batch together?
> 
> Thanks for your input!

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. My concern with reverting just this bit and not the original PR is that we'll leave the original functionality as-is permanently despite the design concerns with it being off-by-default functionality. WDYT?

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