[clang] [Headers][X86] Add macro descriptions to ia32intrin.h (PR #78613)

[Paul T Robinson via cfe-commits]

pogo59 wrote:

Reviewer questions:
1. When a macro is merely an alternate name for an intrinsic function, does it want to be documented as its own intrinsic function? I assume yes in this patch, but if there's a different tactic that avoids duplicating a bunch of descriptions, I'm open to suggestions.
2. Is _bswap (one underscore) a mistake? The [12.0 release notes](https://releases.llvm.org/12.0.0/tools/clang/docs/ReleaseNotes.html#x86-support-in-clang) say it has two underscores.
4. Is _rdtsc correct, or should it be _rdtscp? I have to change the macro anyway, it was wrong.


https://github.com/llvm/llvm-project/pull/78613