Rename DIBuilder::createNullPtrType to createUnspecifiedType and introduce
a zero-argument createNullPtrType function for creating the canonical
nullptr type.

Differential Revision: http://llvm-reviews.chandlerc.com/D1050
Use the zero-argument DIBuilder::createNullPtrType in Clang.

Differential Revision: http://llvm-reviews.chandlerc.com/D1051