Sema: handle `wint_t` more carefully for printf checking

In the case that `win_t` is an `unsigned short` (e.g. on Windows), we would
previously incorrectly diagnose the conversion because we would immediately
promote the argument type from `wint_t` (aka `unsigned short`) to `int` before
checking if the type matched.  This should repair the Windows hosted bots.

llvm-svn: 342565