Make struct frame_arg self-managing

Make struct frame_arg self-managing

This changes struct frame_arg to be self-managing and then fixes the
various users.

Tested by the buildbot.

gdb/ChangeLog
2019-08-07  Tom Tromey  <tromey@adacore.com>

	* stack.c (print_frame_arg, read_frame_local, read_frame_arg)
	(print_frame_args): Update.
	* python/py-framefilter.c (py_print_single_arg, enumerate_args):
	Update.
	* mi/mi-cmd-stack.c (list_arg_or_local): Update.
	* frame.h (struct frame_arg): Add initializers.
	<error>: Now a unique_xmalloc_ptr.