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

[amyk]

Summary

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.

[amyk]

Ping.

[daltenty]

This doesn't appear to be true for 64-bit AIX:

extern "C" int printf(const char *, ...);
int main() {
        printf("wchar_t: %ld\nunsigned short: %ld\n",sizeof(wchar_t),sizeof(unsigned short));
        return 0;
}
$ clang++ -m64 foo.cc
$ ./a.out
wchar_t: 4
unsigned short: 2

[amyk]

Addressed comment made by @daltenty.
Update the patch to account set the wchar type to unsigned short only on 32-bit AIX.

clang/test/Sema/wchar.c

 // RUN: %clang_cc1 %s -fsyntax-only -verify
 // RUN: %clang_cc1 %s -fsyntax-only -fwchar-type=short -fno-signed-wchar -verify -DSHORT_WCHAR
 
 typedef __WCHAR_TYPE__ wchar_t;
 
 #if defined(_WIN32) || defined(_M_IX86) || defined(__CYGWIN__) \
- || defined(_M_X64) || defined(__ORBIS__) || defined(SHORT_WCHAR)
+ || defined(_M_X64) || defined(__ORBIS__) || defined(SHORT_WCHAR) \
+ || defined(_AIX)
   #define WCHAR_T_TYPE unsigned short
 #elif defined(__aarch64__)
   // See AArch64TargetInfo constructor -- unsigned on non-darwin non-OpenBSD non-NetBSD.
   #if defined(__OpenBSD__) || defined(__APPLE__) || defined(__NetBSD__)
     #define WCHAR_T_TYPE int
   #else
     #define WCHAR_T_TYPE unsigned int
   #endif