[clang] [clang][CodeGen] Trigger AST lookup for libfunction builtins (PR #209997)

[Takuto Ikuta via cfe-commits]

atetubou wrote:

> The patch looks better. But if possible, we really hope we can understand the underlying reasons so that we can make the software better.

Posted investigation summary in https://github.com/llvm/llvm-project/issues/209989#issuecomment-4999842627. Let me know if you want to know more detail.

https://github.com/llvm/llvm-project/pull/209997