[clang] [clang-tools-extra] [llvm] [clang][modules] stdarg.h and stddef.h shouldn't directly declare anything (PR #90676)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> I think I like this as a solution, although I wonder if __stdarg_guard_macro.h would be a better name. The name now somewhat implies that it contains the macros that stddef and stdarg define.

But the macros don't actually guard anything. _macro isn't the best name in the world though, maybe _header_macro.h is ok even though they aren't traditional "header macros"?

https://github.com/llvm/llvm-project/pull/90676