Substring's _persistentContent has a fast path that avoids a copy when
the Substring represents the entire outer String. It correctly
detected situations where the wrapped _StringCore was not itself a
slice, but was omitting the extra checks to make sure the Substring's
own range covered the entire String.
Test added.
* Use _ephemeralString for substring comparison
Add back != for String/Substring comparison
* OK fine, Linux, if you're going to be difficult
* Different forms of comparison, tests
* [stdlib] Fix String.UTF16View index sharing
* [stdlib] Fix String.UnicodeScalarView index sharing
* [stdlib] Fix String.CharacterView index sharing
* [stdlib] Test advancing string indices past their ends
* [stdlib] Simplify CharacterView ranged subscript
This decreases total testing time by over a minute on my old Mac Pro.
It probably has much less effect on systems with fewer cores, but shouldn't
be any worse there.
Swift SVN r22745
