Add back UTF-32 for now and fix a MSVC build issue

source/common/sharedobject.h

 /*
 ******************************************************************************
 * Copyright (C) 2015, International Business Machines
 * Corporation and others.  All Rights Reserved.
 ******************************************************************************
 * sharedobject.h
 */
 
 #ifndef __SHAREDOBJECT_H__
 #define __SHAREDOBJECT_H__
 
 
 #include "unicode/uobject.h"
 #include "umutex.h"
 
 U_NAMESPACE_BEGIN
 
 /**
  * Base class for unified cache exposing enough methods to SharedObject
  * instances to allow their addRef() and removeRef() methods to
  * update cache metrics. No other part of ICU, except for SharedObject,
  * should directly call the methods of this base class.
  */
-class UnifiedCacheBase : public UObject {
+class U_COMMON_API UnifiedCacheBase : public UObject {
 public:
     UnifiedCacheBase() { }
 
     /**
      * Called by addRefWhileHoldingCacheLock() when the hard reference count
      * of its instance goes from 0 to 1.
      */
     virtual void incrementItemsInUse() const = 0;
 
     /**
@@ skipping 190 matching lines @@
     mutable u_atomic_int32_t hardRefCount;
     mutable const UnifiedCacheBase *cachePtr;
     void addRef(UBool withCacheLock) const;
     void removeRef(UBool withCacheLock) const;
 
 };
 
 U_NAMESPACE_END
 
 #endif