[AIX] Define WCHAR_T_TYPE as unsigned int on 64-bit AIX for wchar.c test

[Jake-Egan]

Summary

The default wchar type on 64-bit AIX is unsigned int, so this patch sets WCHAR_T_TYPE to unsigned int rather than int.

This patch follows similar reasoning to D110428 except for 64-bit.

[daltenty]

LGTM, thanks!

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(_AIX) && !defined(__64BIT__))
   #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
-#elif defined(__arm) || defined(__MVS__)
+#elif defined(__arm) || defined(__MVS__) || (defined(_AIX) && defined(__64BIT__))
   #define WCHAR_T_TYPE unsigned int
 #elif defined(__sun)
   #if defined(__LP64__)
     #define WCHAR_T_TYPE int
   #else
     #define WCHAR_T_TYPE long
   #endif
 #else /* Solaris, Linux, non-arm64 macOS, ... */