Move the default template arguments into the forward declarations for the containers: deque, forwardlist and list. References PR#22605.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@229705 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/list b/include/list
index 13f8a53..13e1199 100644
--- a/include/list
+++ b/include/list
@@ -226,7 +226,7 @@
     _Tp __value_;
 };
 
-template <class _Tp, class _Alloc> class _LIBCPP_TYPE_VIS_ONLY list;
+template <class _Tp, class _Alloc = allocator<_Tp> > class _LIBCPP_TYPE_VIS_ONLY list;
 template <class _Tp, class _Alloc> class __list_imp;
 template <class _Tp, class _VoidPtr> class _LIBCPP_TYPE_VIS_ONLY __list_const_iterator;
 
@@ -799,7 +799,7 @@
 #endif
 }
 
-template <class _Tp, class _Alloc = allocator<_Tp> >
+template <class _Tp, class _Alloc /*= allocator<_Tp>*/>
 class _LIBCPP_TYPE_VIS_ONLY list
     : private __list_imp<_Tp, _Alloc>
 {