C++ify badness_vector, fix leaks

badness_vector is currently an open coded vector.  This reimplements
it as a std::vector.

This fixes a few leaks as well:

 - find_oload_champ is leaking every badness vector calculated bar the
   one returned.

 - bv->rank is always leaked, since callers of rank_function only
   xfree the badness_vector pointer, not bv->rank.

gdb/ChangeLog:
2018-11-21  Pedro Alves  <palves@redhat.com>

	* gdbtypes.c (compare_badness): Change type of parameters to const
	reference.  Adjust to badness_vector being a std::vector now.
	(rank_function): Adjust to badness_vector being a std::vector now.
	* gdbtypes.h (badness_vector): Now a typedef to std::vector.
	(LENGTH_MATCH): Delete.
	(compare_badness): Change type of parameters to const reference.
	(rank_function): Return a badness_vector by value now.
	(find_overload_match): Adjust to badness_vector being a
	std::vector now.  Remove cleanups.
	(find_oload_champ_namespace): 'oload_champ_bv' parameter now a
	badness_vector pointer.
	(find_oload_champ_namespace_loop): 'oload_champ_bv' parameter now
	a badness_vector pointer.  Adjust to badness_vector being a
	std::vector now.  Remove cleanups.
	(find_oload_champ): 'oload_champ_bv' parameter now
	a badness_vector pointer.  Adjust to badness_vector being a
	std::vector now.  Remove cleanups.