Generator observes a snapshot of the collection. Mutating the
collection should not have any effect on existing generators.
rdar://19726013
Swift SVN r25171
...into the validation suite. This is the wrong solution but at least
the bots will continue to run all the tests and we won't regress.
Swift SVN r24934
This fast path is only taken when the code is specialized.
Fixes <rdar://problem/19331717> Optimizer breaks Set<T> and Dictionary<K, V>
Swift SVN r24479
