attribs: Don't crash on NULL TREE_TYPE in diag_attr_exclusions [PR114634]

attribs: Don't crash on NULL TREE_TYPE in diag_attr_exclusions [PR114634]

The enumerator still doesn't have TREE_TYPE set but diag_attr_exclusions
assumes that all decls must have types.
I think it is better in something as unimportant as diag_attr_exclusions
to be more robust, if there is no type, it can just diagnose exclusions
on the DECL_ATTRIBUTES, like for types it only diagnoses it on
TYPE_ATTRIBUTES.

2024-04-15  Jakub Jelinek  <jakub@redhat.com>

	PR c++/114634
	* attribs.c (diag_attr_exclusions): Set attrs[1] to NULL_TREE for
	decls with NULL TREE_TYPE.

	* g++.dg/ext/attrib68.C: New test.

(cherry picked from commit 7ec54f5fdfec298812a749699874db4d6a7246bb)