Use thread_info_ref in stop_context

This changes stop_context to use a thread_info_ref, removing some
manual reference counting.

gdb/ChangeLog
2020-12-11  Tom Tromey  <tom@tromey.com>

	* infrun.c (struct stop_context) <thread>: Now a thread_info_ref.
	(stop_context::stop_context): Update.
	(stop_context::~stop_context): Remove.