Change all_root_varobjs to take a function_view

Change all_root_varobjs to take a function_view

This changes all_root_varobjs to take a function_view.  This
simplifies some of the callers, in particular we can remove a data
type that only existed to be passed through.

gdb/ChangeLog
2020-12-11  Tom Tromey  <tom@tromey.com>

	* varobj.h (all_root_varobjs): Take a function_view.
	* varobj.c (all_root_varobjs): Take a function_view.
	(varobj_invalidate_iter): Remove unused parameter.
	(varobj_invalidate): Update.
	* mi/mi-cmd-var.c (struct mi_cmd_var_update): Remove.
	(mi_cmd_var_update_iter): Change parameters.