Add four to SwiftVersion

[marcelofabri]

This will allow rules like #1714 to be implemented.

Credits to @ornithocoder for the idea of using #if swift.

[SwiftLintBot]

	1 Warning
⚠️	Please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

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* Add four to SwiftVersion.  
  [marcelofabri](https://github.com/marcelofabri)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

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[codecov-io]

Codecov Report

Merging #1716 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1716   +/-   ##
=======================================
  Coverage   87.43%   87.43%           
=======================================
  Files         208      208           
  Lines       10291    10291           
=======================================
  Hits         8998     8998           
  Misses       1293     1293

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[jpsim]

Does the same approach not work for #if swift(>=3.2)?

Also, kind of a shame that SourceKit itself can't be used to query the Swift version it was built with. There's a "protocol version" request you can make, but that's always been 1.0: https://github.com/apple/swift/blob/master/tools/SourceKit/docs/Protocol.md#protocol-version

[marcelofabri]

Does the same approach not work for #if swift(>=3.2)?

Yes, but that is also true for Swift 4. Since they're the same compiler, just with different modes, I don't even know how SourceKit deals with that (maybe it doesn't?).

There's a "protocol version" request you can make, but that's always been 1.0

Yeah, I've tried that too 😢

[jpsim]

Anyway, 👍