HLSL: struct splitting: assignments of hierarchical split types

This commit adds support for copying nested hierarchical types of split
types.  E.g, a struct of a struct containing both user and builtin interstage
IO variables.

When copying split types, if any subtree does NOT contain builtin interstage
IO, we can copy the whole subtree with one assignment, which saves a bunch
of AST verbosity for memberwise copies of that subtree.

hlsl/hlslGrammar.cpp

@@ -478,7 +478,7 @@ bool HlslGrammar::acceptFullySpecifiedType(TType& type)
     // type_specifier
     if (! acceptType(type)) {
         // If this is not a type, we may have inadvertently gone down a wrong path
-        // py parsing "sample", which can be treated like either an identifier or a
+        // by parsing "sample", which can be treated like either an identifier or a
         // qualifier.  Back it out, if we did.
         if (qualifier.sample)
             recedeToken();