Issue Description:

Swift 3.2 and later incorrectly claims that a generic parameter isn't used in the function signature if the signature uses the generic parameter's associated type.

Example:

protocol Foo {
  associatedtype Bar
}

func foo<T: Foo>(x: T.Bar) -> T.Bar {
  return x
}

Swift 3.1 happily accepts this. Swift 3.2 and later claims

error: generic parameter 'T' is not used in function signature