Issue Description:

Before Swift 3, operators were declared with this syntax:

infix operator <=> {
  associativity right
  precedence 150
}

Now, they are declared using precedence groups, like so:

precedencegroup SpaceshipPrecedence {
  associativity: right
  higherThan: ComparisonPrecedence
}

infix operator <=> : SpaceshipPrecedence

It would be nice to have a Fix-It that converts the old-style declaration to a new-style. It wouldn't be possible to deduce 'higherThan' from a numeric precedence, but you could get the user most of the way there.