Runtime: Fix problems with nil-to-nil casts

We have a special rule that Optional<T>.none successfully dynamically casts
to Optional<U>.none for any T and U. However the implementation was incorrect
if the source and destination types had a different size. We would initialize
the source to nil, and then copy to the result.

The correct implementation is to initialize the result using the result
payload type directly, and not call _succeed() at all.

Fixes <https://bugs.swift.org/browse/SR-1056>.

stdlib/public/runtime/Casting.cpp

@@ -2036,10 +2036,16 @@ checkDynamicCastFromOptional(OpaqueValue *dest,
       _fail(src, srcType, targetType, flags);
       return {false, nullptr};
     }
+
+    // Get the destination payload type
+    const Metadata *targetPayloadType =
+      cast<EnumMetadata>(targetType)->getGenericArgs()[0];
+
     // Inject the .none tag
-    swift_storeEnumTagSinglePayload(dest, payloadType, enumCase,
+    swift_storeEnumTagSinglePayload(dest, targetPayloadType, enumCase,
                                     1 /*emptyCases=*/);
-    _succeed(dest, src, srcType, flags);
+
+    // We don't have to destroy the source, because it was nil.
     return {true, nullptr};
   }
   // .Some