Patterns, parse empty record type annotation

[scheglov]

See co19/LanguageFeatures/Patterns/parenthesized_A01_t01, code like:

  var (() c) = empty;

[scheglov]

Also

String test(Record record) {
  return switch (record) {
    (: final Record case7 as ()) => "case7 = $case7",
    _ => "default"
  };
}

[scheglov]

and

    case (: final () case11?):
      return "case11 = $case11";

[stereotype441]

I have verified that all the examples in this issue are now correctly parsed. But there was a gap in testing (we didn't have any parser tests of variable patterns whose type was a record pattern). I'll follow up with a CL that tests that scenario.