Issue Description:

If I have protocol Old with associatedtype OldType, I may want to define protocol New with associatedtype NewType that inherits Old and provides NewType for OldType. My first attempt at this fails unexpectedly:

protocol Old {
    associatedtype OldType
}
protocol New: Old {
    associatedtype NewType
    typealias OldType = NewType // Warning
}
class Impl: New { // Error
    typealias NewType = String
}

There's both a warning:

Typealias overriding associated type 'OldType' from protocol 'Old' is better expressed as same-type constraint on the protocol

and Impl fails to compile with

Type 'Impl' does not conform to protocol 'New' Type 'Impl' does not conform to protocol 'Old'

This appears to work:

protocol Old {
    associatedtype OldType
}
protocol New: Old where OldType == NewType {
    associatedtype NewType
}
extension New {
    typealias OldType = NewType // Warning
}
class Impl: New {
    typealias NewType = String
}

except there is the same warning as above in the extension and the extension itself should be unnecessary. That makes me think the first approach should work, and the warning is incorrect since there is already a where clause on the protocol.