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

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

> Rebased on top of the fix now; now this is supposed to pass.
> 
> The test header contains a number of references to libc++ still - I'm open to suggestions on either rewriting it, removing parts that may be less relevant here, or just keeping it as is.

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

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