Prevent nullptr_t-typed exprs from being replaced

The Use-Nullptr transform was replacing nullptr_t-typed expressions because in
the AST such expressions have an implicit NullToPointer cast around them. Now
the transform ignores these expressions.

Fixes PR15414.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@177168 91177308-0d34-0410-b5e6-96231b3b80d8