Return std::string from ada_exception_catchpoint_cond_string

Return std::string from ada_exception_catchpoint_cond_string

This changes ada_exception_catchpoint_cond_string to return a
std::string, allowing for the removal of a cleanup in
create_excep_cond_exprs.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	* ada-lang.c (create_excep_cond_exprs): Update.
	(ada_exception_catchpoint_cond_string): Use std::string.