[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 :) )

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@339214 91177308-0d34-0410-b5e6-96231b3b80d8