More fallout from r58501: primary fix is some more corrections to make

More fallout from r58501: primary fix is some more corrections to make
the types for size_t and ptrdiff_t more accurate.  I think all of these
are correct, but please compare the defines for __PTRDIFF_TYPE__ and
__SIZE_TYPE__ to gcc to double-check; this particularly applies to
those on BSD variants, since I'm not sure what they do here; I assume
here that they're the same as on Linux.

Fixes wchar_t to be "int", not "unsigned int" (which I think is
correct on everything but Windows).

Fixes ptrdiff_t to be "int" rather than "short" on PIC16; "short" is an
somewhat strange choice because it normally gets promoted, and it's not
consistent with the choice for size_t.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@58556 91177308-0d34-0410-b5e6-96231b3b80d8