[clang] [Clang][CodeGen] Lookup standard library declarations for builtins to… (PR #204792)

[James Y Knight via cfe-commits]

jyknight wrote:

Agree that this doesn't seem to make sense; `__builtin_xyz` should not (and, AFAIK does not?) depend on whether a function `xyz` was declared ahead-of-time or not.

https://github.com/llvm/llvm-project/pull/204792