Basic: correct `__WINT_TYPE__` on Windows

Windows uses `unsigned short` for `wint_t`.  Correct the type definition as
vended by the compiler.  This type is defined in corecrt.h and is
unconditionally typedef'ed.  cl does not have an equivalent to `__WINT_TYPE__`
which is why this was never detected.

llvm-svn: 342557