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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185114 91177308-0d34-0410-b5e6-96231b3b80d8