btrace: temporarily set inferior_ptid in record_btrace_start_replaying

btrace: temporarily set inferior_ptid in record_btrace_start_replaying

Get_current_frame uses inferior_ptid.  In record_btrace_start_replaying,
we need to get the current frame of the argument thread.  So far, this
has always been inferior_ptid.  With non-stop, this is not guaranteed.

Temporarily set inferior_ptid to the ptid of the argument thread.

We already temporarily set the argument thread's executing flag to false.

Move both into a new function get_thread_current_frame that does the temporary
adjustments, calls get_current_frame, and restores the previous values.

gdb/
	* record-btrace.c (get_thread_current_frame): New.
	(record_btrace_start_replaying): Call get_thread_current_frame.