Detect and throw on a class of bad regexes that we mistakenly accepted before. Thanks to Trevor Smigiel for the report

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@243030 91177308-0d34-0410-b5e6-96231b3b80d8

include/regex

@@ -4305,6 +4305,14 @@ basic_regex<_CharT, _Traits>::__parse_atom(_ForwardIterator __first,
                 }
             }
             break;
+        case '*':
+        case '+':
+        case '?':
+        case '{':
+#ifndef _LIBCPP_NO_EXCEPTIONS
+            throw regex_error(regex_constants::error_badrepeat);
+#endif
+            break;
         default:
             __first = __parse_pattern_character(__first, __last);
             break;