[AIX] Define __THW_BIG_ENDIAN__ macro

[Jake-Egan]

Summary

This patch defines the macro __THW_BIG_ENDIAN__ for AIX.

Tested with SPEC.

[cebowleratibm]

LGTM.

clang/lib/Basic/Targets/OSTargets.h

 template <typename Target>
 class AIXTargetInfo : public OSTargetInfo<Target> {
 protected:
   void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
                     MacroBuilder &Builder) const override {
     DefineStd(Builder, "unix", Opts);
     Builder.defineMacro("_IBMR2");
     Builder.defineMacro("_POWER");
+    Builder.defineMacro("__THW_BIG_ENDIAN__");
 
     Builder.defineMacro("_AIX");
     Builder.defineMacro("__TOS_AIX__");
 
     if (Opts.C11) {
       Builder.defineMacro("__STDC_NO_ATOMICS__");
       Builder.defineMacro("__STDC_NO_THREADS__");
     }

clang/test/Preprocessor/init-ppc.c

 // PPC-AIX:#define __SIZEOF_PTRDIFF_T__ 4
 // PPC-AIX:#define __SIZEOF_SHORT__ 2
 // PPC-AIX:#define __SIZEOF_SIZE_T__ 4
 // PPC-AIX:#define __SIZEOF_WCHAR_T__ 2
 // PPC-AIX:#define __SIZEOF_WINT_T__ 4
 // PPC-AIX:#define __SIZE_MAX__ 4294967295UL
 // PPC-AIX:#define __SIZE_TYPE__ long unsigned int
 // PPC-AIX:#define __SIZE_WIDTH__ 32
+// PPC-AIX:#define __THW_BIG_ENDIAN__ 1
 // PPC-AIX:#define __TOS_AIX__ 1
 // PPC-AIX:#define __UINT16_C_SUFFIX__
 // PPC-AIX:#define __UINT16_MAX__ 65535
 // PPC-AIX:#define __UINT16_TYPE__ unsigned short
 // PPC-AIX:#define __UINT32_C_SUFFIX__ U
 // PPC-AIX:#define __UINT32_MAX__ 4294967295U
 // PPC-AIX:#define __UINT32_TYPE__ unsigned int
 // PPC-AIX:#define __UINT64_C_SUFFIX__ ULL