HLSL: Support the constructor idiom "(struct type)0".

This highly leverages the previous commit to handle partial initializers.

hlsl/hlslParseHelper.h

@@ -172,6 +172,7 @@ protected:
     void declareArray(const TSourceLoc&, TString& identifier, const TType&, TSymbol*&, bool track);
     TIntermNode* executeInitializer(const TSourceLoc&, TIntermTyped* initializer, TVariable* variable);
     TIntermTyped* convertInitializerList(const TSourceLoc&, const TType&, TIntermTyped* initializer);
+    bool HlslParseContext::isZeroConstructor(const TIntermNode*);
     TOperator mapAtomicOp(const TSourceLoc& loc, TOperator op, bool isImage);
 
     // Return true if this node requires L-value conversion (e.g, to an imageStore).