[SDK] Replace NSRange.toRange() with failable initialiser on Range<Int>

[karwa]

Fixes ABI FIXME #75

[slavapestov]

"initializer"

[parkera]

cc @phausler

[parkera]

Please make sure to get review from someone from Foundation for changes in the overlay.

[karwa]

Updated a comment on String.UTF16View

Sure, I'll make sure to request a review from one of you next time :). I just happened to notice that this ABI FIXME was implementable now.

[karwa]

pinging @phausler

[phausler]

This is an API change and would make swift-corelibs deviate from the Darwin versions

[parkera]

This is in the overlay, actually. I think we should consider it for post-Swift 4.

[karwa]

The ABI FIXME led me to believe this source-breaking change was always planned when the requisite compiler features were available. It's just about when we force users to change their code. If the design is settled, I'd advocate for getting the "definitive" API out there as soon as possible.

[tkremenek]

This pull request was opened 22 months ago. Is this still worth keeping open? There has been no movement here, and this PR cannot be merged as is into master.

[CodaFi]

The bulk of this has been subsumed by the new Range(_:in:) constructor. I'm going to close this. If there is any more movement or you would like to revisit this, please reopen this pull request and rebase it onto master @karwa.