[Win32] Overload ==, != for locale_t and long long

Summary:
_is_chartype_l (needed for isxdigit_l) in MinGW compares locale_t and NULL.
NULL is 'long long' for 64-bit, and this results in ambiguous overloads when
compiled with Clang.  Define a concrete overload for the operators to fix the
ambiguity.

Reviewers: mstorsjo, EricWF, srhines, danalbert

Subscribers: christof, cfe-commits, ldionne

Differential Revision: https://reviews.llvm.org/D48749

llvm-svn: 336141