gdb/continuations: turn continuation functions into inferior methods

gdb/continuations: turn continuation functions into inferior methods

Turn continuations-related functions into methods of the inferior
class.  This is a refactoring.

gdb/ChangeLog:
2021-04-22  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* Makefile.in (COMMON_SFILES): Remove continuations.c.
	* inferior.c (inferior::add_continuation): New method, adapted
	from 'add_inferior_continuation'.
	(inferior::do_all_continuations): New method, adapted from
	'do_all_inferior_continuations'.
	(inferior::~inferior): Clear the list of continuations directly.
	* inferior.h (class inferior) <continuations>: Rename into...
	<m_continuations>: ...this and make private.
	* continuations.c: Remove.
	* continuations.h: Remove.
	* event-top.c: Don't include "continuations.h".

	Update the users below.
	* inf-loop.c (inferior_event_handler)
	* infcmd.c (attach_command)
	(notice_new_inferior): Update.