cobol: Tweak adjustments to location_t of GENERIC nodes for PERFORM.

COBOL has a group of PERFORM statements that require careful adjustments to
the location_t elements of the GENERIC nodes so that the COBOL-aware version
of GDB behaves properly.  These changes are in service of that goal.

gcc/cobol/ChangeLog:

	* genapi.cc (leave_procedure): Adjust location_t for PERFORM.
	(parser_perform_times): Likewise.
	(internal_perform_through_times): Likewise.
	(perform_outofline_before_until): Likewise.
	(perform_outofline_after_until): Likewise.
	(perform_outofline_testafter_varying): Likewise.
	(perform_outofline_before_varying): Likewise.

(cherry picked from commit 3cc2116fe6694b0e52855427fc1fd1335c6d00b6)