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

test/std/containers/container.adaptors/stack/stack.defn/emplace.pass.cpp

@@ -12,22 +12,31 @@
 // <stack>
 
 // template <class... Args> reference emplace(Args&&... args);
+// return type is 'reference' in C++17; 'void' before
 
 #include <stack>
 #include <cassert>
 
+#include "test_macros.h"
+
 #include "../../../Emplaceable.h"
 
 int main()
 {
     typedef Emplaceable T;
     std::stack<Emplaceable> q;
+#if TEST_STD_VER > 14
     T& r1 = q.emplace(1, 2.5);
     assert(&r1 == &q.top());
     T& r2 = q.emplace(2, 3.5);
     assert(&r2 == &q.top());
     T& r3 = q.emplace(3, 4.5);
     assert(&r3 == &q.top());
+#else
+    q.emplace(1, 2.5);
+    q.emplace(2, 3.5);
+    q.emplace(3, 4.5);
+#endif
     assert(q.size() == 3);
     assert(q.top() == Emplaceable(3, 4.5));
 }