[clang] [clang][Sema] Fix crash on atomic builtins with incomplete type args (PR #96374)

[Takuya Shimizu via cfe-commits]

hazohelet wrote:

Thanks for the feedback. `RequireCompleteType` seems like the right direction.
I'm not writing about the crash fix to the release note because this crash was introduced AFTER clang 18 was released.


https://github.com/llvm/llvm-project/pull/96374