HLSL: Implement basic "struct" grammar.

Test/hlsl.struct.frag

@@ -0,0 +1,26 @@
+struct {
+};
+
+struct {
+    bool b;
+};
+
+struct myS {
+    bool b, c;
+    float4 a, d;
+};
+
+myS s1;
+
+struct {
+    float4 i;
+} s2;
+
+float4 PixelShaderFunction(float4 input) : COLOR0
+{
+    struct FS {
+        bool3 b3;
+    } s3;
+
+    s3 == s3;
+}