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

[Nick Sarnie via cfe-commits]

sarnex wrote:

Sure, this was an interesting one. I thought I was losing my mind until I actually got into the debugger and saw the problem.

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