Issue Description:

When I inherit a protocol adding mutating keyword to the property with the same name from the parent protocol, I can't create a concrete type even there's no compile error for the protocols.
Check out this code:

protocol FooProtocol {
    var value: Int { get }
}
protocol MutatingFooProtocol: FooProtocol {
    var value: Int { mutating get }
}
// no compile error so far.
struct Bar: MutatingFooProtocol {
    private var storage: Int

    var value: Int {
        mutating get {
            self.storage += 1
            return self.storage
        }
    }
}

This produces compile error that Bar is not conforming to protocol FooProtocol. but adding non mutating var value: Int does not solve the problem. It creates another compile error `Invalid redeclaration of value.

I think inheriting same property name with adding mutating attribute should not be allowed from the first place.