Annotate swtich fall-through

The clang default to warning for missing fall-through and it does
not support all comment-like annotation that gcc does.  Use C23
[[fallthrough]] annotation instead.
proper attribute instead.

Reviewed-by: Collin Funk <collin.funk1@gmail.com>

stdio-common/printf-parsemb.c

@@ -272,7 +272,7 @@ __parse_one_specmb (const UCHAR_T *format, size_t posn,
 	if (*format != L_('l'))
 	  break;
 	++format;
-	/* FALLTHROUGH */
+	[[fallthrough]]		;
       case L_('L'):
 	/* doubles are long doubles, and ints are long long ints.  */
       case L_('q'):

stdio-common/vfscanf-internal.c

@@ -915,7 +915,7 @@ __vfscanf_internal (FILE *s, const char *format, va_list argptr,
 
 	      break;
 	    }
-	  /* FALLTHROUGH */
+	  [[fallthrough]];
 	case L_('C'):
 	  if (width == -1)
 	    width = 1;
@@ -1258,7 +1258,7 @@ __vfscanf_internal (FILE *s, const char *format, va_list argptr,
 		}
 	      break;
 	    }
-	  /* FALLTHROUGH */
+	  [[fallthrough]]		;
 
 	case L_('S'):
 	  {