Use std::vector in add_using_directive

Use std::vector in add_using_directive

This changes add_using_directive to accept a std::vector and then
changes the callers.  This allows removing a cleanup.

ChangeLog
2017-09-09  Tom Tromey  <tom@tromey.com>

	* namespace.h (add_using_directive): Update.
	* namespace.c (add_using_directive): Change type of excludes to
	std::vector.
	* dwarf2read.c (read_import_statement): Use std::vector.
	(read_namespace): Update.
	* cp-namespace.c (cp_scan_for_anonymous_namespaces): Update.