Remove cleanups from dbxread.c

Remove cleanups from dbxread.c

This removes the remaining cleanups from dbxread.c, via std::vector,
scoped_restore, and unique_xmalloc_ptr.

Tested by the buildbot.

ChangeLog
2018-05-25  Tom Tromey  <tom@tromey.com>

	* dbxread.c (init_bincl_list): Remove.
	(bincl_list): Now a std::vector.
	(bincls_allocated, next_bincl): Remove.
	(free_bincl_list, do_free_bincl_list_cleanup)
	(make_cleanup_free_bincl_list): Remove.
	(dbx_read_symtab, elfstab_build_psymtabs): Use scoped_restore,
	unique_xmalloc_ptr.
	(find_corresponding_bincl_psymtab, read_dbx_symtab): Update.
	(struct header_file_location): Add constructor.
	(add_bincl_to_list): Remove.