c++: ICE with using and enum [PR100659]

Here the code for 'using enum' is confused by the combination of a
using-decl and an enum that are not from 'using enum'; this CONST_DECL is
from the normal unscoped enum scoping.

	PR c++/100659

gcc/cp/ChangeLog:

	* cp-tree.h (CONST_DECL_USING_P): Check for null TREE_TYPE.

gcc/testsuite/ChangeLog:

	* g++.dg/parse/access13.C: New test.