HLSL: Make attributes for built-in override semantics.

Test/hlsl.PointSize.geom

 struct S {
-    [[vk::builtin("PointSize")]] float ps;
+    [[vk::builtin("PointSize")]] float ps : PSIZE;
 };
 
 [maxvertexcount(4)]

hlsl/hlslParseHelper.cpp

@@ -5856,7 +5856,8 @@ void HlslParseContext::handleSemantic(TSourceLoc loc, TQualifier& qualifier, TBu
         break;
     }
 
-    qualifier.builtIn = builtIn;
+    if (qualifier.builtIn == EbvNone)
+        qualifier.builtIn = builtIn;
     qualifier.semanticName = intermediate.addSemanticName(upperCase);
 }