[libcxx] [test] Allow a standard library that implements LWG 1203 in istream.rvalue/rvalue.pass.cpp

(Still pending review at https://reviews.llvm.org/D47400 which has been open since may; will ask for forgiveness rather than permission :) )

llvm-svn: 339214

libcxx/test/std/input.output/iostream.format/input.streams/istream.rvalue/rvalue.pass.cpp

@@ -65,7 +65,7 @@ int main()
     { // test perfect forwarding
         assert(called == false);
         std::istringstream ss;
-        auto& out = (std::move(ss) >> A{});
+        auto&& out = (std::move(ss) >> A{});
         assert(&out == &ss);
         assert(called);
     }