Make tdesc_type::name an std::string

Make tdesc_type::name an std::string

This patch makes tdesc_type::name an std::string.  This way, we don't
need to free it manually in ~tdesc_type.  I think the comment on top of
name is not correct, the string is always malloc'ed.

gdb/ChangeLog:

	* target-descriptions.c (struct tdesc_type) <name>: Change type
	to std::string.
	<~tdesc_type>: Don't manually free name.
	<operator==>: Adjust.
	(tdesc_named_type): Adjust.
	(tdesc_find_type): Adjust.
	(tdesc_gdb_type): Adjust.
	(class print_c_tdesc) <visit>: Adjust.