[private-named-parameters] Implement declaring parameters in CFE.

[munificent]

Private named parameters are being implemented concurrently with declaring parameters and the two features interact. A declaring parameter is allowed to be a private named parameter:

class C({final int _x});

main() {
  print(C(x: 1)._x); // 1.
}

Since this part of private named parameters is blocked on declaring parameters being implemented, this issue tracks that implementation separate from the rest of the CFE implementation of private named parameters.