More array casting work:

- Continue adding support for checked downcasts of array types (rdar://problem/16535104)
- Fix non-bridged array conversions post-r17868
- Fix rdar://problem/16773693
- Add tests for NSArray coercions to and from Array<T>

Swift SVN r17957

test/expr/cast/array_downcast.swift

@@ -0,0 +1,24 @@
+// RUN: %swift %s -verify
+
+class V {}
+class U : V {}
+class T : U {}
+
+var v = V()
+var u = U()
+var t = T()
+
+var va = [v]
+var ua = [u]
+var ta = [t]
+
+va = ta
+
+var va2: (V[])? = va as V[]
+var v2: V = va2![0]
+
+var ua2: (U[])? = va as U[]
+var u2: U = ua2![0]
+
+var ta2: (T[])? = va as T[]
+var t2: T = ta2![0]