c++: Fix ICE on g++.dg/modules/adl-3_c.C [PR107379]

As mentioned in the PR, apparently my r13-2887 P1467R9 changes
regressed these tests on powerpc64le-linux with IEEE quad by default.

I believe my changes just uncovered a latent bug.
The problem is that push_namespace calls find_namespace_slot,
which does:
  tree *slot = DECL_NAMESPACE_BINDINGS (ns)
    ->find_slot_with_hash (name, name ? IDENTIFIER_HASH_VALUE (name) : 0,
                           create_p ? INSERT : NO_INSERT);
In the <identifier_node 0x7fffe9f55ac0 details> ns case, slot is non-NULL
above with a binding_vector in it.
Then pushdecl is called and this does:
                  slot = find_namespace_slot (ns, name, ns == current_namespace);
where ns == current_namespace (ns is :: and name is details) is true.
So this again calls
          tree *slot = DECL_NAMESPACE_BINDINGS (ns)
            ->find_slot_with_hash (name, name ? IDENTIFIER_HASH_VALUE (name) : 0,
                                   create_p ? INSERT : NO_INSERT);
but this time with create_p and so INSERT.
At this point we reach
          if (insert == INSERT && m_size * 3 <= m_n_elements * 4)
            expand ();
and when we are unlucky and the occupancy of the hash table just reached 3/4,
expand () is called and the hash table is reallocated.  But when that happens,
it means the slot pointer in the pushdecl caller (push_namespace) points to
freed memory and so any accesses to it in make_namespace_finish will be UB.

The following patch fixes it by calling find_namespace_slot again even if it
was non-NULL, just doesn't assert it is *slot == ns in that case (because
it often is not).

2022-10-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/107379
	* name-lookup.cc (push_namespace): Call find_namespace_slot again
	after pushdecl as the hash table might be expanded during pushdecl.