Add character set related __STDC_* definitions.

Clang uses UTF-16 and UTF-32 for its char16_t's and char32_t's
exclusively. This means that we can define __STDC_UTF_16__ and
__STDC_UTF_32__ unconditionally.

While there, define __STDC_MB_MIGHT_NEQ_WC__ for FreeBSD. FreeBSD's
wchar_t's don't encode characters as ISO-10646; the encoding depends on
the locale used. Because the character set used might not be a superset
of ASCII, we must define __STDC_MB_MIGHT_NEQ_WC__.

llvm-svn: 191631

clang/lib/Frontend/InitPreprocessor.cpp

@@ -330,6 +330,14 @@ static void InitializeStandardPredefinedMacros(const TargetInfo &TI,
       Builder.defineMacro("__cplusplus", "199711L");
   }
 
+  // In C11 these are environment macros. In C++11 they are only defined
+  // as part of <cuchar>. To prevent breakage when mixing C and C++
+  // code, define these macros unconditionally. We can define them
+  // unconditionally, as Clang always uses UTF-16 and UTF-32 for 16-bit
+  // and 32-bit character literals.
+  Builder.defineMacro("__STDC_UTF_16__", "1");
+  Builder.defineMacro("__STDC_UTF_32__", "1");
+
   if (LangOpts.ObjC1)
     Builder.defineMacro("__OBJC__");