Fix -Wshadow warnings and enable warnings by default for C++ >= 11

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@288564 91177308-0d34-0410-b5e6-96231b3b80d8

include/forward_list

diff --git a/include/forward_list b/include/forward_list
index f344d2e..a75d6e9 100644
--- a/include/forward_list
+++ b/include/forward_list
@@ -876,8 +876,9 @@
 
 #if _LIBCPP_STD_VER > 11
 template <class _Tp, class _Alloc>
-forward_list<_Tp, _Alloc>::forward_list(size_type __n, const allocator_type& __a)
-    : base ( __a )
+forward_list<_Tp, _Alloc>::forward_list(size_type __n,
+                                        const allocator_type& __base_alloc)
+    : base ( __base_alloc )
 {
     if (__n > 0)
     {