[clang] [clang][Sema] Fix and reapply 'Declare builtins used in #pragma intrinsic #138205' (PR #142019)

[Reid Kleckner via cfe-commits]

https://github.com/rnk approved this pull request.

Wow. Thanks for debugging it! I assumed this had more to do with intrin0.h than the namespaces in the libc++ string header.

https://github.com/llvm/llvm-project/pull/142019