Restore (un-revert) sting comparison, with fixes
More exhaustive testing of opaque strings, which consistently reproduces prior sporadic failure. Shims fixups. Some test tweaking.
The simplest way to keep the string buffer alive is simply to always grab its
iterator, even if we're not going to use it. Thanks @milseman for that idea and
@jckarter for the diagnosis help!
It is still possible to get the same behavior by providing an explicit
type context, or in a generic code, but otherwise, `String("")!` will
not compile.
Now that `String` conforms to the `BidirectionalCollection` protocol, in
the expression `let x = [""].joined()` the best matching overload for
`joined` is no longer the one returning `String`.
Fixes: <rdar://problem/31899440>
* [stdlib] String : RangeReplaceableCollection & BidirectionalCollection
* Add source compatibility hack for Swift.max
* Add source compatibility hack for Swift.min
* Remove redundant conformance in benchmarks
* Fix stupid typo I thought I'd already pushed
* XFAIL testing now-redundant conformance
* XFAIL an IDE test for now
