Use std::string rather than grow_vect

Use std::string rather than grow_vect

This removes the "GROW_VECT" macro and helper function in favor of
simply using std::string in a few spots.

gdb/ChangeLog
2021-03-02  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_fold_name, ada_variant_discrim_name)
	(ada_enum_name, scan_discrim_bound, to_fixed_range_type): Use
	std::string.
	(GROW_VECT): Remove.
	(grow_vect): Remove.