swift234

[stdlib] bugfix: Copy bridged NSMutableArray's

When NSArrays are bridged into Array<T>, they weren't being .copy()'d.
As a result, changes to an NSMutableArray (cast to NSArray), could cause
"unrelated" Swift Arrays to mutate.  Found by inspection while
bottlenecking unsafeBitCast's in Array implementation, which also
happens in this commit.

Swift SVN r20678