Change ada_catchpoint::excep_string to be a std::string

Change ada_catchpoint::excep_string to be a std::string

This changes ada_catchpoint::excep_string to be a std::string and then
fixes up all t he users.

This found a memory leak in catch_ada_exception_command_split, where
"cond" was copied but never freed.

I changed the type of the "cond_string" argument to
catch_ada_exception_command_split to follow the rule that out
parameters should be pointers and not references.

This patch enables the removal of some cleanups and also the function
ada_get_next_arg.

ChangeLog
2018-05-21  Tom Tromey  <tom@tromey.com>

	* mi/mi-cmd-catch.c (mi_cmd_catch_assert)
	(mi_cmd_catch_exception, mi_cmd_catch_handlers): Update.
	* ada-lang.h (create_ada_exception_catchpoint): Update.
	* ada-lang.c (struct ada_catchpoint) <excep_string>: Now a
	std::string.
	(create_excep_cond_exprs, ~ada_catchpoint)
	(should_stop_exception, print_one_exception)
	(print_mention_exception, print_recreate_exception): Update.
	(ada_get_next_arg): Remove.
	(catch_ada_exception_command_split): Use std::string.  Change type
	of "excep_string", "cond_string".
	(catch_ada_exception_command): Update.
	(create_ada_exception_catchpoint): Change type of excep_string.
	(ada_exception_sal): Remove excep_string parameter.
	(~ada_catchpoint): Remove.