HLSL: Partially address issue #463: accept GLSL layout(...).

This includes all "per variable" layout qualifiers, but the
key ones mattering and tested for now are:
  set=
  binding=
  constant_id=
  push_constant

hlsl/hlslGrammar.h

@@ -67,7 +67,8 @@ namespace glslang {
         bool acceptSamplerDeclarationDX9(TType&);
         bool acceptSamplerState();
         bool acceptFullySpecifiedType(TType&);
-        void acceptQualifier(TQualifier&);
+        bool acceptQualifier(TQualifier&);
+        bool acceptLayoutQualifierList(TQualifier&);
         bool acceptType(TType&);
         bool acceptTemplateType(TBasicType&);
         bool acceptVectorTemplateType(TType&);