Revert "Remove stored instance properties from NSNumber"

This reverts commit bb182da.

Author: norio-nomura

Foundation/NSNumber.swift

@@ -205,6 +205,9 @@ private struct CFSInt128Struct {
 
 open class NSNumber : NSValue {
     typealias CFType = CFNumber
+    // This layout MUST be the same as CFNumber so that they are bridgeable
+    private var _base = _CFInfo(typeID: CFNumberGetTypeID())
+    private var _pad: UInt64 = 0
 
     internal var _cfObject: CFType {
         return unsafeBitCast(self, to: CFType.self)