C++-ify break-catch-throw

C++-ify break-catch-throw

This changes exception_catchpoint's "exception_rx' member to be a
std::string, and updating the users.

ChangeLog
2017-07-22  Tom Tromey  <tom@tromey.com>

	* break-catch-throw.c (struct exception_catchpoint)
	<exception_rx>: Now a std::string.
	(~exception_catchpoint): Remove.
	(print_one_detail_exception_catchpoint): Update.
	(handle_gnu_v3_exceptions): Change type of except_rx.
	(extract_exception_regexp): Return a std::string.
	(catch_exception_command_1): Update.