Change the return type of emplace_[front|back] back to void when building with C++14 or before. Resolves PR31680.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292990 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/stack b/include/stack
index 228864e..c797ea5 100644
--- a/include/stack
+++ b/include/stack
@@ -55,7 +55,7 @@
 
     void push(const value_type& x);
     void push(value_type&& x);
-    template <class... Args> reference emplace(Args&&... args);
+    template <class... Args> reference emplace(Args&&... args); // reference in C++17
     void pop();
 
     void swap(stack& c) noexcept(is_nothrow_swappable_v<Container>)
@@ -199,8 +199,13 @@
 #ifndef _LIBCPP_HAS_NO_VARIADICS
     template <class... _Args>
         _LIBCPP_INLINE_VISIBILITY
+#if _LIBCPP_STD_VER > 14
         reference emplace(_Args&&... __args)
         { return c.emplace_back(_VSTD::forward<_Args>(__args)...);}
+#else
+        void      emplace(_Args&&... __args)
+        {        c.emplace_back(_VSTD::forward<_Args>(__args)...);}
+#endif
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
     _LIBCPP_INLINE_VISIBILITY