[clang] [clang] [test] Verify that intrinsic headers don't use unreserved names (PR #161817)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> I've no problem with referring to libc++ but it might make sense to add a comment explaining that this should stay in sync with the libc++ reference file

Yeah, some sort of such reference could be reasonable, and could make the origins and context of the checks clearer.

A second issue is that this patch currently only covers the couple of x86/arm intrinsic headers that I manually added the include for; ideally we'd test all clang bundled headers with something like this. It's easy to slip in the `#include` in other tests as well, but it's also easy to accidentally miss some header.

https://github.com/llvm/llvm-project/pull/161817