Implement struct type_stack with a vector

Implement struct type_stack with a vector

This patch changes the home-made stack implementation with a vector,
which makes it a bit more concise and readable.

Regtested on the buildbot.

gdb/ChangeLog:

	* parser-defs.h (struct type_stack) <elements>: Change type to
	std::vector<union type_stack_elt>.
	<depth, size>: Remove.
	* parse.c (parse_exp_in_context_1): Adjust.
	(type_stack_reserve): Remove.
	(check_type_stack_depth): Remove.
	(insert_into_type_stack): Adjust to std::vector.
	(insert_type): Likewise.
	(push_type): Likewise.
	(push_type_int): Likewise.
	(insert_type_address_space): Likewise.
	(pop_type): Likewise.
	(pop_type_int): Likewise.
	(pop_typelist): Likewise.
	(pop_type_stack): Likewise.
	(append_type_stack): Likewise.
	(push_type_stack): Likewise.
	(get_type_stack): Likewise.
	(type_stack_cleanup): Likewise.
	(push_typelist): Likewise.
	(follow_types): Likewise.
	(_initialize_parse): Likewise.