Remove release_stop_context_cleanup

Remove release_stop_context_cleanup

This removes release_stop_context_cleanup, replacing it with a
stop_context destructor.  It also mildly c++-ifies this struct.

gdb/ChangeLog
2018-09-17  Tom Tromey  <tom@tromey.com>

	* infrun.c (struct stop_context): Declare constructor,
	destructor, "changed" method.
	(stop_context::stop_context): Rename from save_stop_context.
	(stop_context::~stop_context): Rename from
	release_stop_context_cleanup.
	(normal_stop): Update.
	(stop_context::changed): Rename from stop_context_changed.  Return
	bool.