More use of std::vector in linespec.c

More use of std::vector in linespec.c

This changes some spots in linespec.c to take a std::vector.  This
patch spilled out to objc-lang.c a bit as well.  This change allows
for the removal of some cleanups.

ChangeLog
2018-04-05  Tom Tromey  <tom@tromey.com>

	* utils.c (compare_strings): Remove.
	* utils.h (compare_strings): Remove.
	* objc-lang.h (find_imps): Update.
	* objc-lang.c (find_methods): Take a std::vector.
	(uniquify_strings, find_imps): Likewise.
	* linespec.c (find_methods): Take a std::vector.
	(decode_objc): Use std::vector.
	(add_all_symbol_names_from_pspace, find_superclass_methods): Take
	a std::vector.
	(find_method, find_function_symbols): Use std::vector.