AST: Have type deepCopy() preserve type graphs as graphs.

Previously, a type graph would turn into a type tree. That is,
a deep node that is shared would have multiple copies made.

This is important when creating IO and non-IO versions of deep types.

Test/hlsl.typeGraphCopy.vert

@@ -0,0 +1,24 @@
+struct N1 {
+    int a;
+    float b;
+};
+
+struct N2 {
+    N1 s1;
+    N1 s2;
+};
+
+struct N3 {
+    N2 t1;
+    N1 t2;
+    N2 t3;
+};
+
+typedef N3 T3;
+
+T3 foo;
+
+float main()
+{
+    return foo.t3.s2.b;
+}