Return a vector from ada_lookup_symbol_list

Return a vector from ada_lookup_symbol_list

This changes ada_lookup_symbol_list to return a std::vector, and
changes various other helper functions to follow.  This simplifies the
code, and makes it more type-safe (by using a vector where an obstack
had been used).

gdb/ChangeLog
2021-03-02  Tom Tromey  <tromey@adacore.com>

	* ada-lang.h (ada_lookup_symbol_list): Return a vector.
	* ada-lang.c (resolve_subexp): Update.
	(ada_resolve_function): Accept a vector.
	(is_nonfunction, add_defn_to_vec)
	(add_symbols_from_enclosing_procs): Likewise.
	(num_defns_collected, defns_collected): Remove.
	(remove_extra_symbols): Return a vector.
	(remove_irrelevant_renamings): Return void.
	(ada_add_local_symbols): Accept a vector.
	(struct match_data) <obstackp>: Remove.
	<resultp>: New member.
	(aux_add_nonlocal_symbols): Update.
	(ada_add_block_renamings, add_nonlocal_symbols)
	(ada_add_all_symbols): Accept a vector.
	(ada_lookup_symbol_list_worker, ada_lookup_symbol_list): Return a
	vector.
	(ada_lookup_symbol): Update.
	(ada_add_block_symbols): Accept a vector.
	(get_var_value, iterate_over_symbols): Update.
	* ada-exp.y (block_lookup, write_var_or_type, write_name_assoc):
	Update.