Added String.init<T>(_: T) and marked it unavailable

[harlanhaskins]

What's in this pull request?

Adds back the String.init<T>(_: T) initializer with a hint to either conform to LosslessStringConvertible or use String.init(describing:)

Resolved bug number: (SR-2228)

---

Before merging this pull request to apple/swift repository:

• Test pull request on Swift continuous integration.

Triggering Swift CI

The swift-ci is triggered by writing a comment on this PR addressed to the GitHub user @swift-ci. Different tests will run depending on the specific comment that you use. The currently available comments are:

Smoke Testing

Platform	Comment
All supported platforms	@swift-ci Please smoke test
All supported platforms	@swift-ci Please smoke test and merge
OS X platform	@swift-ci Please smoke test OS X platform
Linux platform	@swift-ci Please smoke test Linux platform

A smoke test on macOS does the following:

1. Builds the compiler incrementally.
2. Builds the standard library only for macOS. Simulator standard libraries and
   device standard libraries are not built.
3. lldb is not built.
4. The test and validation-test targets are run only for macOS. The optimized
   version of these tests are not run.

A smoke test on Linux does the following:

1. Builds the compiler incrementally.
2. Builds the standard library incrementally.
3. lldb is built incrementally.
4. The swift test and validation-test targets are run. The optimized version of these
   tests are not run.
5. lldb is tested.

Validation Testing

Platform	Comment
All supported platforms	@swift-ci Please test
All supported platforms	@swift-ci Please test and merge
OS X platform	@swift-ci Please test OS X platform
OS X platform	@swift-ci Please benchmark
Linux platform	@swift-ci Please test Linux platform

Lint Testing

Language	Comment
Python	@swift-ci Please Python lint

Note: Only members of the Apple organization can trigger swift-ci.

[harlanhaskins]

@swift-ci please smoke test

[jrose-apple]

Should this have a renamed: field as well, to add the fix-it?

[harlanhaskins]

I wondered that too, because I think that's the most obvious fix, but I was hesitant to introduce a fix-it along with an alternative.

[jrose-apple]

@dabrahams, what do you think?

[harlanhaskins]

Spoke with Dave offline. Rename, no message.

[harlanhaskins]

@swift-ci please smoke test and merge

[lattner]

This should provide a fixit hint to do the renaming, right? If so, please add a test for it. Add {{1-2=}} to the end of the expected-error line and run the test. The test will fail and tell you what the right thing is. Thanks!

[jrose-apple]

No, see above.

[harlanhaskins]

It should provide the fixit hint, but it's actually not providing it right now. 🤔
@CodaFi and I are investigating.

[jrose-apple]

Oh, I totally read that resolution backwards. My bad.

[CodaFi]

Oh lldb test suite, why are you being so silly today.

@swift-ci please smoke test Linux platform.

[gribozavr]

@swift-ci Please smoke test

[gribozavr]

@harlanhaskins Could you make a PR for the swift-3.0-branch?