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

lib/SILPasses/GenericSpecializer.cpp

@@ -278,6 +278,7 @@ private:
     switch (Kind) {
     case CheckedCastKind::Unresolved:
     case CheckedCastKind::Coercion:
+    case CheckedCastKind::ArrayDowncast:
       llvm_unreachable("invalid for SIL");
 
     // These are not affected by specialization.
@@ -397,6 +398,7 @@ private:
     switch (Kind) {
     case CheckedCastKind::Unresolved:
     case CheckedCastKind::Coercion:
+    case CheckedCastKind::ArrayDowncast:        
       llvm_unreachable("invalid for SIL");
 
     // These are not affected by specialization.