lib/Basic/Targets.cpp: Fix __declspec() on Mingw-w64.

It should be defined as-is. Some headers would detect existence of __declspec and use one.

llvm-svn: 123690

clang/lib/Basic/Targets.cpp

@@ -1622,7 +1622,7 @@ public:
     DefineStd(Builder, "WIN64", Opts);
     Builder.defineMacro("__MSVCRT__");
     Builder.defineMacro("__MINGW64__");
-    Builder.defineMacro("__declspec");
+    Builder.defineMacro("__declspec", "__declspec");
   }
 };
 } // end anonymous namespace