Return std::string from ada_encode

This changes ada_encode to return a std::string.  This simplifies it
somewhat, removes a use of GROW_VECT, and is also simpler for callers
to use.

gdb/ChangeLog
2020-10-09  Tom Tromey  <tromey@adacore.com>

	* ada-lang.h (ada_encode): Return std::string.
	* ada-lang.c (ada_encode_1): Return std::string.
	(ada_encode): Likewise.
	(type_from_tag, ada_lookup_name_info::ada_lookup_name_info):
	Update.
	* ada-exp.y (block_lookup, write_var_or_type): Update.