[clang] [clang][Sema] Declare builtins used in #pragma intrinsic (PR #138205)

[Nick Sarnie via cfe-commits]

sarnex wrote:

@alanzhao1 let me know if this is a big problem for Chrome and I can revert it, I'd prefer to try to fix it quickly first and if I can't then revert it, thanks.

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