[clang] Do not crash on pointer wchar_t pointer assignment.

wchar_t can be signed (thus hasSignedIntegerRepresentation() returns
true), but it doesn't have an unsigned type, which would lead to a crash
when trying to get it.

With this fix, we special-case WideChar types in the pointer assignment
code.

Differential Revision: https://reviews.llvm.org/D91625