Class-fy tdesc_reg tdesc_type and tdesc_feature

Class-fy tdesc_reg tdesc_type and tdesc_feature

This patch class-fies them, adding ctor, dtor, and deleting
copy ctor and assignment operator.

gdb:

2017-06-20  Yao Qi  <yao.qi@linaro.org>

	* target-descriptions.c (tdesc_reg): Add ctor, dtor.
	Delete copy ctor and assignment operator.
	(tdesc_type): Likewise.
	(tdesc_feature): Likewise.
	(tdesc_free_reg): Remove.
	(tdesc_create_reg): Use new.
	(tdesc_free_type): Remove.
	(tdesc_create_vector): Use new.
	(tdesc_create_union): Likewise.
	(tdesc_create_flags): Likewise.
	(tdesc_create_enum): Likewise.
	(tdesc_free_feature): Delete.
	(free_target_description): Use delete.