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

test/std/re/re.regex/re.regex.construct/bad_escape.pass.cpp

@@ -22,7 +22,7 @@ static bool error_escape_thrown(const char *pat)
     bool result = false;
     try {
         std::regex re(pat);
-    } catch (std::regex_error &ex) {
+    } catch (const std::regex_error &ex) {
         result = (ex.code() == std::regex_constants::error_escape);
     }
     return result;