re PR c/90677 (gcc-9.1.0 fails to build __gcc_diag__ souce: error: 'cgraph_node' is not defined as a type)

	PR c/90677
	* cp-objcp-common.c (identifier_global_tag): Return NULL_TREE if name
	has not been found, rather than error_mark_node.

	* c-c++-common/pr90677-2.c: New test.

From-SVN: r279840