[Runtime] Handle Error-conforming-to-NSObject casting fully.

I missed a case where an Error-conforming class is dynamically casted
to NSObject (via NSError). Fix it.

Author: DougGregor

stdlib/public/runtime/Casting.cpp

@@ -1301,7 +1301,8 @@ static bool _dynamicCastToUnknownClassFromExistential(OpaqueValue *dest,
 #if SWIFT_OBJC_INTEROP
     // If we're casting to NSError, we may need a representation change,
     // so fall into the general swift_dynamicCast path.
-    if (targetType == getNSErrorMetadata()) {
+    if (targetType == getNSErrorMetadata() ||
+        targetType == getNSObjectMetadata()) {
       return swift_dynamicCast(dest, src, swift_getObjectType((HeapObject*)obj),
                                targetType, flags);
     }

test/stdlib/ErrorBridged.swift

@@ -785,6 +785,11 @@ ErrorBridgingTests.test("error-to-NSObject casts") {
 
     // "is" check
     expectTrue(error is NSObject)
+
+    // Unconditional cast to a dictionary.
+    let dict = ["key" : NoisyError()]
+    let anyOfDict = dict as AnyObject
+    let dict2 = anyOfDict as! [String: NSObject]
   }
 }