[libcxx][regex] add [[clang::fallthrough]] to suppress fallthrough warning

Author: vegerot

libcxx/include/regex

@@ -3921,7 +3921,7 @@ _ForwardIterator basic_regex<_CharT, _Traits>::__parse_character_escape(
       if (__hd == -1)
         __throw_regex_error<regex_constants::error_escape>();
       __sum = 16 * __sum + static_cast<unsigned>(__hd);
-      // fallthrough
+    [[clang::fallthrough]];
     case 'x':
       ++__first;
       if (__first == __last)