Issue Description:

Original mailing list proposal: https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20161128/003567.html

The idea here is that it would be convenient if, when compiling the Swift runtime and standard library, one could choose whether to compile with Objective-C interop enabled or disabled, for any target.

For example, I should be able to compile for macOS with Objective-C interop disabled. I should be able to compile for Linux with Objective-C interop enabled (although because of Objective-C ABI assumptions baked into Swift, there's no guarantee that would work – but I'd prefer to deal with that issue later).