Validate implicit subscript get

[marcelofabri]

Fixes #898

This depends on swiftlang/swift#11258 (which is already available on latest development snapshots).

I don't know how should we proceed with this one. Should we merge right away? Or wait for that PR to be on an official release?

This also needs a CHANGELOG entry.

[SwiftLintBot]

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

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Here's an example of your CHANGELOG entry:

* Validate implicit subscript get.  
  [marcelofabri](https://github.com/marcelofabri)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

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[jpsim]

Let's revisit when Swift 4.1 is out?

[marcelofabri]

In theory we could make it already available for people using preview toolchains, but I guess no one actually does that. I'll close it by now, but keep the branch alive.