Issue Description:

private extension Foo {
    fileprivate func foo() {} // warning: declaring a fileprivate instance method in a private extension
}

The source of the problem probably lies in the fact that extensions are top level declarations, for which private and fileprivate are equivalent.
foo() obviously can't be more than fileprivate and will be so by default, so we can safely omit its access level here. The warning, however, seems to be unnecessary (we have full right to have foo explicitly fileprivate) and ill-advised (it offers a fix-it that downgrades foo to private)