__split_buffer should only require default constructible.  Bug found and fixed by Jared Hoberock

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@138726 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__split_buffer b/include/__split_buffer
index 73d9f35..dbf3ce0 100644
--- a/include/__split_buffer
+++ b/include/__split_buffer
@@ -208,7 +208,7 @@
     __alloc_rr& __a = this->__alloc();
     do
     {
-        __alloc_traits::construct(__a, _VSTD::__to_raw_pointer(this->__end_), value_type());
+        __alloc_traits::construct(__a, _VSTD::__to_raw_pointer(this->__end_));
         ++this->__end_;
         --__n;
     } while (__n > 0);