[Type decoder] Generalize the canonicalization of nominal type decl references.

Various TypeDecoder clients will depend on having the "bare" nominal
type declaration demangled node for looking up nominal type descriptors,
so move the generic argument-stripping code into TypeDecoder.

include/swift/Reflection/TypeRefBuilder.h

@@ -185,6 +185,8 @@ public:
     return TR;
   }
 
+  Demangle::NodeFactory &getNodeFactory() { return Dem; }
+
   ///
   /// Factory methods for all TypeRef kinds
   ///