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/compile/compile-cplus-symbols.c

@@ -132,7 +132,7 @@ convert_one_symbol (compile_cplus_instance *instance,
 		     "be referenced from the current thread in "
 		     "compiled code."),
 		   sym.symbol->print_name ());
-	  /* FALLTHROUGH */
+	  [[fallthrough]];
 	case LOC_UNRESOLVED:
 	  /* 'symbol_name' cannot be used here as that one is used only for
 	     local variables from compile_dwarf_expr_to_c.