[C++11] Switch the CMake option from LLVM_ENABLE_CXX11 (default on) to
LLVM_ENABLE_CXX1Y (default *off*). =D C++98 is dead. Long live C++11.

I don't exactly recommend using C++1y just yet though...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202567 91177308-0d34-0410-b5e6-96231b3b80d8