Issue Description:

When adopting Equatable in a struct, Swift usually synthesizes Equatable conformance if all struct members are Equatable as well. However, this seems to stop working when declaring a member as weak var or unowned var:

// Error: Type 'Foo' does not conform to protocol 'Equatable'
struct Foo: Equatable {
    weak var x: Bar? // Removing `weak` fixes above error, type `Bar?` is fine
    var y: String
}

class Bar: Equatable {
    static func == (lhs: Bar, rhs: Bar) -> Bool {
        return true
    }
}

Expected behaviour:

Swift should synthesize Equatable conformance for a type when all its members conform to Equatable, regardless of the members' storage types (unowned or weak).

(Related forum thread: https://forums.swift.org/t/struct-with-weak-member-prevents-synthesized-equatable/19778)