When Clang defines _MSC_VER, prefer Clang versions of attribute macros (#516)

Author: jbatez

include/cpp2util.h

@@ -681,7 +681,7 @@ class out {
 //
 //-----------------------------------------------------------------------
 //
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang_major__)
     #define CPP2_FORCE_INLINE        __forceinline
     #define CPP2_FORCE_INLINE_LAMBDA [[msvc::forceinline]]
     #define CPP2_LAMBDA_NO_DISCARD