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.

gtests/Hlsl.FromFile.cpp

@@ -235,6 +235,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.tx.bracket.frag", "main"},
         {"hlsl.type.half.frag", "main"},
         {"hlsl.type.identifier.frag", "main"},
+        {"hlsl.typeGraphCopy.vert", "main"},
         {"hlsl.typedef.frag", "PixelShaderFunction"},
         {"hlsl.whileLoop.frag", "PixelShaderFunction"},
         {"hlsl.void.frag", "PixelShaderFunction"},