Fix handling of wint_t - we can't assume wint_t is purely an integer promotion of...

Fix handling of wint_t - we can't assume wint_t is purely an integer promotion of wchar_t - they may differ in signedness.

Teach ASTContext about WIntType, and have it taken from TargetInfo like WCharType. Should fix test/Sema/format-strings.c for ARM, with the exception of one subtest which will fail if wint_t and wchar_t are the same size and wint_t is signed, wchar_t is unsigned.

There'll be a followup commit to fix that.

Reviewed by Chandler and Hans at http://llvm.org/reviews/r/8

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156165 91177308-0d34-0410-b5e6-96231b3b80d8