Add missing symtab for functions added by using declarations

Fixes fully qualified names for functions added by using declarations:
- Error messages show fully qualified names in Lua
- Overload warning messages show fully qualified names
- Error messages calling dispatch functions for handling overloaded methods in OCaml, Python and Tcl

Examples/test-suite/errors/cpp_using_declaration_overload.stderr

@@ -1,2 +1,2 @@
 cpp_using_declaration_overload.i:7: Warning 512: Overloaded method Derived::m(bool) const ignored,
-cpp_using_declaration_overload.i:8: Warning 512: using non-const method m(bool) instead.
+cpp_using_declaration_overload.i:8: Warning 512: using non-const method Derived::m(bool) instead.

Source/Modules/typepass.cxx

@@ -1093,6 +1093,9 @@ class TypePass:private Dispatcher {
 		      Setattr(nn, "access", Getattr(n, "access"));
 		      if (!Getattr(nn, "sym:name"))
 			Setattr(nn, "sym:name", symname);
+		      Symtab *st = Getattr(n, "sym:symtab");
+		      assert(st);
+		      Setattr(nn, "sym:symtab", st);
 
 		      if (!GetFlag(nn, "feature:ignore")) {
 			ParmList *parms = CopyParmList(Getattr(c, "parms"));