[clang] [clang][Sema] Don't warn for implicit uses of builtins in system headers (PR #138205)

[Artem Belevich via cfe-commits]

Artem-B wrote:

Something does not add up here. AFAICT, using builtins w/o explicitly declaring them is something that's done all the time. https://godbolt.org/z/ha47W53dh

In that sense, we should not be needing to filter out the diagnostics coming from the system headers only. There should not be any diagnostics for those builtins at all, from anywhere. 

I must be missing something here. @rnk -- is the requirement for builtin declarations a windows-specific quirk?

https://github.com/llvm/llvm-project/pull/138205