[Algorithm] Add make_const_ref corresponding to make_const_ptr

Reviewers: aaron.ballman

Subscribers: dexonsmith, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D56622

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351009 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/STLExtras.h

diff --git a/include/llvm/ADT/STLExtras.h b/include/llvm/ADT/STLExtras.h
index 8685f0e..f66ca7c 100644
--- a/include/llvm/ADT/STLExtras.h
+++ b/include/llvm/ADT/STLExtras.h
@@ -75,6 +75,12 @@
   using type =
       typename std::add_pointer<typename std::add_const<T>::type>::type;
 };
+
+template <typename T> struct make_const_ref {
+  using type = typename std::add_lvalue_reference<
+      typename std::add_const<T>::type>::type;
+};
+
 //===----------------------------------------------------------------------===//
 //     Extra additions to <functional>
 //===----------------------------------------------------------------------===//