Target: fix wchar_t definition for Windows on ARM

Windows on ARM uses AAPCS, but has some deviations.  wchar_t remains an unsigned
short on WoA, which does not conform to AAPCS.  Ensure that wchar_t is defined
accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207929 91177308-0d34-0410-b5e6-96231b3b80d8