[ptr-traits] Sink a constructor definition to the .cpp file and add

missing includes so that the pointee types for DenseMap pointer keys and
such are complete prior to us querying the pointer traits for them.

This is part of a series of patches to allow LLVM to check for complete
pointee types when computing its pointer traits. This is absolutely
necessary to get correct (or reproducible) results for things like how
many low bits are guaranteed to be zero.

llvm-svn: 256550