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

[Martin Storsjö via cfe-commits]

mstorsjo 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.

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