c++: Fix up ICE when cplus_decl_attributes is called with error_mark_node attributes [PR104668]

cplus_decl_attributes can be called with attributes equal to
error_mark_node, there are some spots in the function that test
it or decl_attributes it calls starts with:
  if (TREE_TYPE (*node) == error_mark_node || attributes == error_mark_node)
    return NULL_TREE;
But the recent PR104245 change broke this when processing_template_decl
is true.

The patch returns early for attributes error_mark_node from
cplus_decl_attributes.

2022-04-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104668
	* decl2.cc (splice_template_attributes): Return NULL if *p is
	error_mark_node.
	(cplus_decl_attributes): Return early if attributes is
	error_mark_node.  Don't check that later.

	* g++.dg/cpp0x/pr104668.C: New test.