On the following testcase GCC ICEs, because last_decl is error_mark_node,
and diag_attr_exclusions assumes that if it is not NULL, it must be a decl.

The following patch just doesn't diagnose attribute exclusions if the
other decl is erroneous (and thus we've already reported errors for it).

2020-04-23  Jakub Jelinek  <jakub@redhat.com>

	PR c/94705
	* attribs.c (decl_attribute): Don't diagnose attribute exclusions
	if last_decl is error_mark_node or has such a TREE_TYPE.

	* gcc.dg/pr94705.c: New test.