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

[Phoebe Wang via cfe-commits]

https://github.com/phoebewang commented:

> 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.

I think single underscore is correct, it conform the difination in Intrinsic Guide.

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