HLSL: Add tests and refine what decorations are passed through per stage/in/out.

Test/hlsl.structIoFourWay.frag

@@ -0,0 +1,18 @@
+struct T {
+    float f : packoffset(c4.y);    // artificial, but validates all different treatments: uniform offset
+    centroid float g;              // interpolant input
+    float d: SV_DepthGreaterEqual; // fragment output
+    float4 normal;                 // non-IO
+};
+
+T s;  // loose uniform
+
+cbuffer buff {
+    T t : packoffset(c5.z);
+};
+
+T main(T t : myInput) : SV_Target0
+{
+    T local;
+    return local;
+}