Add the test for libstdc++ versions newer than 4.6 so we don't
accidentally pick that up while using Clang and run into subtle bugs
down the road related to C++11 features not fully implemented in that
version of the standard library.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199484 91177308-0d34-0410-b5e6-96231b3b80d8