re PR c/71573 (ICE on invalid C code on x86_64-linux-gnu (tree check: expected function_decl, have var_decl in implicitly_declare))

	PR c/71573
	* c-decl.c (implicitly_declare): Return decl early not only for
	error_mark_nodes, but for anything that is not a FUNCTION_DECL.

	* gcc.dg/noncompile/pr71573.c: New test.

From-SVN: r238885