Format string warnings: don't a.k.a. wchar_t with wchar_t.

This fixes the case where Clang would output:
 error: format specifies type 'wchar_t *' (aka 'wchar_t *')

ArgTypeResult::getRepresentativeTypeName needs to take into account
that wchar_t can be a built-in type (as opposed to in C, where it is a
typedef).

llvm-svn: 149387