[AIX] Define WCHAR_T_TYPE as unsigned short on AIX for wchar.c test case.

The default wchar type is different on AIX vs. Linux. When this test is run on
AIX, WCHAR_T_TYPE ends up being set to int. This is incorrect as the default
wchar type on AIX is actually unsigned short, and setting the type incorrectly
causes the expected errors to not be found.

This patch sets the type correctly (to unsigned short) for AIX.

Differential Revision: https://reviews.llvm.org/D110428