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

This highly leverages the previous commit to handle partial initializers.

Test/hlsl.partialInit.frag

@@ -8,15 +8,22 @@ struct outs {
 static float4 gv = {0,0,1};
 static float gfa[3] = {0,0};
 
+struct Nest {
+    float4x3 m;
+    outs os;
+    bool b;
+};
+
 outs PixelShaderFunction(float4 input) : COLOR0
 {
     outs o2 = { 3 };
     outs o4;
     o4.v = gv * gfa[2];
     outs o1 = { };
-//    outs o3 = (outs)0;
-//    o4 = (outs)0;
+    outs o3 = (outs)0;
+    o4 = (outs)0;
     o4.c = o1.c;
+    Nest nest = (Nest)0;
 
     return o4;
 }