Improve enum element pattern matching diagnostics when the arity of arguments mismatch

[hamishknight]

For the following:

func foo(_ r: Result<String, Error>) {
  let _ = {
    switch r {
    case .success:
      "a"
    case .failure():
      "b"
    }
  }
}

we emit the error:

error: type '()' cannot conform to 'Error' [type_cannot_conform]
    case .failure():
          ^

The non-closure case isn't much better

func foo(_ r: Result<String, Error>) {
  switch r {
  case .success:
    break
  case .failure(): // error: tuple pattern cannot match values of the non-tuple type 'any Error'
    break
  }
}

We ought to improve the diagnostics around enum element pattern argument matching to give diagnostics comparable to existing argument list diagnostics.

rdar://72846034