Revert "[ConstraintSystem] Fix the ordering of functions with optional parame…"

[rudkx]

Reverts #17855

It turns out this needs a bit more work to avoid source compatibility problems.

For example we consider the self.x in the interpolation below to be ambiguous with these changes whereas we previously didn't.

class C {
  var x: Int?
  func x(_ y: Int) {}
  func x(_ y: Float) {
    let _ = "\(String(describing: self.x))"
  }
}

[rudkx]

@swift-ci Please smoke test and merge

[xedin]

Thank you! I was so hopeful that it'll help me to merge my changes to cache but I guess I'll have to wait a bit longer...