Add a constructor and destructor to linespec_result

Add a constructor and destructor to linespec_result

linespec_result is only ever allocated on the stack, so it's
relatively easy to convert to having a constructor and a destructor.
This patch makes this change.  This removes some cleanups.

gdb/ChangeLog
2017-04-12  Tom Tromey  <tom@tromey.com>

	* linespec.h (struct linespec_result): Add constructor and
	destructor.
	(init_linespec_result, destroy_linespec_result)
	(make_cleanup_destroy_linespec_result): Don't declare.
	* linespec.c (init_linespec_result): Remove.
	(linespec_result::~linespec_result): Rename from
	destroy_linespec_result.  Update.
	(cleanup_linespec_result, make_cleanup_destroy_linespec_result):
	Remove.
	* breakpoint.c (create_breakpoint, break_range_command)
	(decode_location_default): Update.
	* ax-gdb.c (agent_command_1): Update.