Fix NATVIS for llvm::PointerIntPair

We don't need to cast to the type of the template argument because it
is always an unsigned number. When passed an enumeration type, MSVC's
debug formatter would get confused, so this fixes issues like looking
at a llvm::PointerIntPair<const ContentCache *, 3, CharacteristicKind>
such as what's used in SrcMgr::FileInfo.