Correctly set the FunctionRefKind for enum element patterns

[hamishknight]

Currently we set FunctionRefKind::Compound for EnumElementPatterns to avoid needing to match labels in the solver. We instead ought to be setting the right kind and properly handling the label matching in the solver.

While here it also looks like the compound naming needs fixing too, e.g we currently allow this:

struct S: Equatable {
  static func bar(x: Int) -> Self { fatalError() }
  public static func == (_: Self, _: Self) -> Bool { false }
}

func foo(_ x: S) {
  _ = {
    switch x {
    case .bar(x:)(x: 0):
      break
    default:
      break
    }
  }
}

rdar://140415944