Use C++17 [[fallthrough]] attribute

This changes gdb to use the C++17 [[fallthrough]] attribute rather
than special comments.

This was mostly done by script, but I neglected a few spellings and so
also fixed it up by hand.

I suspect this fixes the bug mentioned below, by switching to a
standard approach that, presumably, clang supports.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23159
Approved-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Luis Machado <luis.machado@arm.com>
Approved-By: Pedro Alves <pedro@palves.net>

gdb/cli/cli-setshow.c

@@ -379,7 +379,7 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
     case var_filename:
       if (*arg == '\0')
 	error_no_arg (_("filename to set it to."));
-      /* FALLTHROUGH */
+      [[fallthrough]];
     case var_optional_filename:
       {
 	char *val = NULL;