tree-object-size: Abstract object_sizes array

Put all accesses to object_sizes behind functions so that we can add
dynamic capability more easily.

gcc/ChangeLog:

	* tree-object-size.c (object_sizes_grow, object_sizes_release,
	object_sizes_unknown_p, object_sizes_get, object_size_set_force,
	object_sizes_set): New functions.
	(addr_object_size, compute_builtin_object_size,
	expr_object_size, call_object_size, unknown_object_size,
	merge_object_sizes, plus_stmt_object_size,
	cond_expr_object_size, collect_object_sizes_for,
	check_for_plus_in_loops_1, init_object_sizes,
	fini_object_sizes): Adjust.

Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>