Restore Note About Optionality Mismatch in Redeclared Functions Involving IUOs and Optionals

We currently consider functions that attempt to overload with an implicitly unwrapped optional and a plain optional to be redeclarations of each other and emit an error

func test1(i: Int?) {} // 'test1(i:)' previously declared here
func test1(i: Int!) {} // error: invalid redeclaration of 'test1(i:)'

There used to be a note in Swift 4.1 about how these redeclarations only differed in optionality, but it was removed here https://github.com/apple/swift/commit/f08823757ae39ef7b6ad5b1eb2d7f50942b3f689

We should really add it back, it was helpful.