Issue Description:

protocol P {
    associatedtype AT
    typealias T = UnsafePointer<AT>

    var a: T { get }
}
public struct S<AT>: P {
    // var a: Self.T // ok
    var a: Self.T { fatalError() }
    let b: UnsafePointer<T>
}

Result:
Assertion failed: (GenericSig && "Dependent type in pattern without generic signature?"), function requiresClass, file /Users/buildnode/jenkins/workspace/oss-swift-package-osx/swift/lib/SIL/AbstractionPattern.cpp, line 231.