[C++1y] Don't error if the user is enabling C++1y, which is *newer* than
C++11. I'm not sure that this is a good idea, but I know some crazy
folks on the core working group who like to live dangerously, and they
should still be able to build LLD. =D

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@202568 91177308-0d34-0410-b5e6-96231b3b80d8