WIP: HLSL: Support InputPatch variables in patch constant functions

Previously, patch constant functions only accepted OutputPatch.  This
adds InputPatch support, via a pseudo-builtin variable type, so that
the patch can be tracked clear through from the qualifier.

hlsl/hlslParseHelper.h

@@ -386,6 +386,7 @@ protected:
     };
 
     TMap<tInterstageIoData, TVariable*> interstageBuiltInIo; // individual builtin interstage IO vars, indexed by builtin type.
+    TVariable* inputPatch;
 
     // We have to move array references to structs containing builtin interstage IO to the split variables.
     // This is only handled for one level.  This stores the index, because we'll need it in the future, since