* c-decl.c (lookup_label): Return NULL_TREE instead of 0.

	* c-decl.c (lookup_label): Return NULL_TREE instead of 0.
	(lookup_tag): Change the type of THISLEVEL_ONLY to bool.
	Return NULL_TREE instead of 0.
	(lookup_name): Return NULL_TREE instead of 0.
	(lookup_name_in_scope): Likewise.
	(shadow_tag_warned): Use true instead of 1 and NULL_TREE instead of 0.
	(parser_xref_tag): Use false instead of 0.
	(start_struct): Use true instead of 1.
	(start_enum): Use true instead of 1 and NULL_TREE instead of 0.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@227791 138bc75d-0d04-0410-961f-82ee72b054a4