PrintAsClang: Ignore non-null attributes in C output mode

In C mode we may still print _Nonnull attributes. Don't let clang warn
on them and ignore the attribute if the C compiler doesn't know it.

Author: xymus

lib/PrintAsClang/PrintAsClang.cpp

@@ -207,6 +207,15 @@ static void writePrologue(raw_ostream &out, ASTContext &ctx,
 
   static_assert(SWIFT_MAX_IMPORTED_SIMD_ELEMENTS == 4,
               "need to add SIMD typedefs here if max elements is increased");
+
+  // Ignore nullability attributes for C clients.
+  out << "#if !defined(__OBJC__) && !defined(__cplusplus)\n"
+         "# if __has_feature(nullability)\n"
+         "#  pragma clang diagnostic ignored \"-Wnullability-extension\"\n"
+         "# else\n"
+         "#  define _Nonnull\n"
+         "# endif\n";
+         "#endif\n";
 }
 
 static int compareImportModulesByName(const ImportModuleTy *left,