Fix const correctness bug in __move_assign.  Found and fixed by Ion Gaztañaga.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@139032 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__split_buffer b/include/__split_buffer
index dbf3ce0..d5b8f0a 100644
--- a/include/__split_buffer
+++ b/include/__split_buffer
@@ -141,14 +141,14 @@
 
 private:
     _LIBCPP_INLINE_VISIBILITY
-    void __move_assign_alloc(const __split_buffer& __c, true_type)
+    void __move_assign_alloc(__split_buffer& __c, true_type)
         _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
         {
             __alloc() = _VSTD::move(__c.__alloc());
         }
 
     _LIBCPP_INLINE_VISIBILITY
-    void __move_assign_alloc(const __split_buffer& __c, false_type) _NOEXCEPT
+    void __move_assign_alloc(__split_buffer& __c, false_type) _NOEXCEPT
         {}
 
     _LIBCPP_INLINE_VISIBILITY