[swift-evolution] Idea: Support #if os(Darwin) as shorthand for os(iOS) || os(OSX) || os(watchOS) || os(tvOS)

[Kevin Ballard]

I'd like to see #if support os(Darwin) as shorthand for any Apple platform. This is a lot shorter to type, and it's also future-compatible if Apple ever releases another Darwin-based platform. If os(Darwin) evaluates to true, then `import Darwin` should always work, and presumably `import Foundation` as well.

Without this, I think people are going to be tempted to write `if !os(Linux)` instead of writing out all 4 Apple platforms, and this is unfortunate because it makes the assumption that Linux is the only non-Apple platform, and that's simply not true.

-Kevin Ballard