Make tdesc_type::u::u::fields an std::vector

Make tdesc_type::u::u::fields an std::vector

This patch makes the tdesc_type::u::u::fields an std::vector of
tdesc_type_field.   The difficulty here is that the vector is part of a
union.  Because of this, I made fields a pointer to a vector, and
instantiate/destroy the vector if the type is one that uses this member
of the union

The field tdesc_type_field::name is changed to an std::string at the
same time.

gdb/ChangeLog:

	* target-descriptions.c (tdesc_type_field): Remove typedef.
	(DEF_VEC_O (tdesc_type_field)): Remove.
	(struct tdesc_type_field): Add constructor.
	<name>: Change type to std::string.
	(struct tdesc_type) <tdesc_type>: Instantiate vector if the type
	kind uses it.
	<~tdesc_type>: Destroy vector if the type kind uses it.
	<u::u::fields>: Change type to std::vector.
	(tdesc_gdb_type): Adjust.
	(tdesc_add_field): Adjust.
	(tdesc_add_typed_bitfield): Adjust.
	(tdesc_add_field): Adjust.
	(tdesc_add_enum_value): Adjust.
	(class print_c_tdesc) <visit>: Adjust.