Const qualify __mem_fn call operator

QOI improvement.

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

llvm-svn: 199848
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: a5adf489b74dc8cc90340d0c304e30346dcf5cf2
diff --git a/include/functional b/include/functional
index d40f70a..891ed46 100644
--- a/include/functional
+++ b/include/functional
@@ -1221,7 +1221,7 @@
     template <class... _ArgTypes>
        _LIBCPP_INLINE_VISIBILITY
        typename __invoke_return<type, _ArgTypes...>::type
-          operator() (_ArgTypes&&... __args)
+          operator() (_ArgTypes&&... __args) const
           {
               return __invoke(__f_, _VSTD::forward<_ArgTypes>(__args)...);
           }