This allows array types to get the best performance. The default
implementation for range replaceable collections appends elements one by
one, and the optimizer can't hoist uniqueness checks from the loop in
that case yet.
Swift SVN r27314
Array(other_collection) is using an optimized code path for copying
collections. An explicit for loop does not.
This commit should recover the performance regression in
rdar://20530390.
Swift SVN r27313
