Make tdesc_feature::types an std::vector

Make tdesc_feature::types an std::vector

This patch makes tdesc_feature::types an std::vector of unique_ptr of
tdesc_type.  This way, we don't need to manually free the objects and
the vector in ~tdesc_feature.

gdb/ChangeLog:

	* target-descriptions.c (tdesc_type_p): Remove typedef.
	(DEF_VEC_P (tdesc_type_p)): Remove.
	(struct tdesc_feature) <types>: Change type to std::vector.
	<~tdesc_feature>: Replace with default implementation.
	<accept>: Adjust.
	(tdesc_named_type): Adjust.
	(tdesc_create_vector): Adjust.
	(tdesc_create_struct): Adjust.
	(tdesc_create_union): Adjust.
	(tdesc_create_flags): Adjust.
	(tdesc_create_enum): Adjust.