c++: Fix ICE with invalid using enum [PR96462]

c++: Fix ICE with invalid using enum [PR96462]

Here we ICE in finish_nonmember_using_decl -> lookup_using_decl ->
... -> find_namespace_slot because "name" is not an IDENTIFIER_NODE.
It is a BIT_NOT_EXPR because this broken test uses

  using E::~E; // SCOPE::NAME

A using-decl can't refer to a destructor, and lookup_using_decl already
checks that in the class member case.  But in C++17, we do the "enum
scope is the enclosing scope" block, and so scope gets set to ::, and
we go into the NAMESPACE_DECL block.  In C++20 we don't do it, we go
to the ENUMERAL_TYPE block.

I resorted to hoisting the check along with a diagnostic tweak: we
don't want to print "::::~E names destructor".

gcc/cp/ChangeLog:

	PR c++/96462
	* name-lookup.c (lookup_using_decl): Hoist the destructor check.

gcc/testsuite/ChangeLog:

	PR c++/96462
	* g++.dg/cpp2a/using-enum-8.C: New test.