46: Restricting cross-module struct initializers

Proposal: https://github.com/apple/swift-evolution/blob/master/proposals/0189-restrict-cross-module-struct-initializers.md

Implementation: https://github.com/apple/swift/pull/12834

Forum accepted announcement: http://forums.swift.org/t/accepted-se-0189-restrict-cross-module-struct-initializers/7164

Mailing list discussion: https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20171120/041478.html