Make ada-lang.c::add_angle_brackets return an std::string

Make ada-lang.c::add_angle_brackets return an std::string

This removes the need for manual memory management.  It may also be a
bit more efficient, since the returned string can be moved all the way
into the destination, in ada_lookup_name_info::matches.

gdb/ChangeLog:

	* ada-lang.c (add_angle_brackets): Return std::string.