Issue Description:

protocol Proto {
    var test: Int? { get set }
}
struct Struct: Proto {
    var test: Int! = 123
}

This code will give you an error like this:

main.swift:6:8: error: type 'Struct' does not conform to protocol 'Proto'
struct Struct: Proto {
       ^
main.swift:7:9: note: candidate has non-matching type 'Int?'
    var test: Int! = 123
        ^
main.swift:4:9: note: protocol requires property 'test' with type 'Int?'; do you want to add a stub?
    var test: Int? { get set }
        ^

I'm talking specifically about the "non-matching type 'Int?'" note. It's silly to say that `Int?` doesn't match itself. Swift should decide if `Int?` and `Int!` are the same type, or different types.

If they are the same type, the code should compile.
If they are not the same type, then the error message should say the correct type (i.e. `Int!`)

The problem happens also with unannotated obj-c classes, which makes the problem harder to notice, because it's not immediately obvious that `@property NSNumber *foo;` isn't an optional when compiler tells you it is.