__cxa_demangle: Fix constructor cv qualifier handling

[tberghammer]

Summary

Previously if we parsed a constructor then we set parsed_ctor_dtor_cv
to true and never reseted it. This causes issue when a template argument
references a constructor (e.g. type of lambda defined inside a
constructor) as we will have the parsed_ctor_dtor_cv flag set what will
cause issues when parsing later arguments.

[compnerd]

Can you please ensure that you cross-port this into llvm/lib/Demangle/ItaniumDemangle.cpp?

src/cxa_demangle.cpp

         save_value<decltype(db.tag_templates)> sb(db.tag_templates);
         if (db.encoding_depth > 1)
             db.tag_templates = true;
+        save_value<decltype(db.parsed_ctor_dtor_cv)> sp(db.parsed_ctor_dtor_cv);
+        db.parsed_ctor_dtor_cv = false;
         switch (*first)
         {
         case 'G':

test/test_demangle.pass.cpp

     {"_ZZ2f6vE1b", "f6()::b"},
     {"_ZNV3$_35test9Ev", "$_3::test9() volatile"},
     {"_ZN5Test8I3$_2EC1ES0_", "Test8<$_2>::Test8($_2)"},
+    {"_Z3fooIZN3BarC1EvE3$_0EvT_", "void foo<Bar::Bar()::$_0>(Bar::Bar()::$_0)"},
     {"_ZGVZN1N1gEvE1a", "guard variable for N::g()::a"},
     {"_ZplRK1YRA100_P1X", "operator+(Y const&, X* (&) [100])"},
     {"_Z1fno", "f(__int128, unsigned __int128)"},