[swift-3.0-branch] Added String.init<T>(_: T) and marked it unavailable

[harlanhaskins]

What's in this pull request?

Dual of #3938 for Swift 3.

Resolved bug number: (SR-)

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[harlanhaskins]

@gribozavr

[gribozavr]

LGTM.

[harlanhaskins]

@lattner For some reason, this case just isn't generating a fixit and the fixit test always fails. @CodaFi and I will investigate more tomorrow.

[lattner]

Thanks @harlanhaskins and @CodaFi, the migrator folks will thank you :-)