[clang-tools-extra] [clang-tidy] Allow use of decltype(nullptr) for defining nullptr_t (PR #231637)

[Nikolas Klauser via cfe-commits]

https://github.com/philnik777 commented:

FWIW I don't think we need to modify clang-tidy to avoid warning just for libc++. We can just suppress the warning. We already have similar annotations to suppress `unique_ptr` warnings inside the `unique_ptr` implementation IIRC.

https://github.com/llvm/llvm-project/pull/231637