Need one more swap overload for swapping two lvalue vector<bool>::reference's.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@178016 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__bit_reference b/include/__bit_reference
index 8180295..1621deb 100644
--- a/include/__bit_reference
+++ b/include/__bit_reference
@@ -81,6 +81,16 @@
 {
 };
 
+template <class _Cp>
+_LIBCPP_INLINE_VISIBILITY inline
+void
+swap(__bit_reference<_Cp> __x, __bit_reference<_Cp> __y) _NOEXCEPT
+{
+    bool __t = __x;
+    __x = __y;
+    __y = __t;
+}
+
 template <class _Cp, class _Dp>
 _LIBCPP_INLINE_VISIBILITY inline
 void
diff --git a/test/containers/sequences/vector.bool/swap.pass.cpp b/test/containers/sequences/vector.bool/swap.pass.cpp
index 642641e..5d0f0af 100644
--- a/test/containers/sequences/vector.bool/swap.pass.cpp
+++ b/test/containers/sequences/vector.bool/swap.pass.cpp
@@ -51,4 +51,14 @@
         assert(v1.get_allocator() == A(2));
         assert(v2.get_allocator() == A(1));
     }
+    {
+        std::vector<bool> v(2);
+        std::vector<bool>::reference r1 = v[0];
+        std::vector<bool>::reference r2 = v[1];
+        r1 = true;
+        using std::swap;
+        swap(r1, r2);
+        assert(v[0] == false);
+        assert(v[1] == true);
+    }
 }