Forward declare the swap overload

Author: vogelsgesang

libcxx/include/__exception/exception_ptr.h

@@ -62,6 +62,8 @@ _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 
 #ifndef _LIBCPP_ABI_MICROSOFT
 
+inline _LIBCPP_HIDE_FROM_ABI void swap(exception_ptr& __x, exception_ptr& __y) _NOEXCEPT;
+
 class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
   void* __ptr_;