[clang][NFC] Refactor `Selector` to use `PointerIntPair` inside (#69916)

Refactor `uintptr_t` inside of `clang::Selector` that holds a pointer to `IdentifierInfo` or `MultiKeywordSelector` and `IdentifierInfoFlag` enum into `PointerIntPair`. This is a part of `PointerIntPair` migration outlined in https://github.com/llvm/llvm-project/issues/69835.

Unlike `uintpt_t`, `PointerIntPair` required pointee types to be complete, so I had to shuffle definitions of `MultiKeywordSelector` and `detail::DeclarationNameExtra` around to make them complete at `Selector`. Also, there were outdated specializations of `PointerLikeTypeTraits` for `IdentifierInfo *`, which are no longer needed, because `alignof` that primary template use works just fine. Not just that, but they declared that `IdentifierInfo *` has only 1 spare lower bit, but today they are 8-byte aligned.