Const qualify __mem_fn call operator

QOI improvement.

Differential Revision: http://llvm-reviews.chandlerc.com/D2059

llvm-svn: 199848
diff --git a/libcxx/test/utilities/function.objects/func.memfn/member_data.pass.cpp b/libcxx/test/utilities/function.objects/func.memfn/member_data.pass.cpp
index 048fca4..dff211c 100644
--- a/libcxx/test/utilities/function.objects/func.memfn/member_data.pass.cpp
+++ b/libcxx/test/utilities/function.objects/func.memfn/member_data.pass.cpp
@@ -32,6 +32,8 @@
     assert(a.data_ == 6);
     const A* cap = ap;
     assert(f(cap) == f(ap));
+    const F& cf = f;
+    assert(cf(ap) == f(ap));
     }
 }