[C++11] Switch CMake to use C++11 by default! Next up, autoconf/make!

Now, please don't get too excited. I've just toggled the default to suss
out the last remaining bot problems. This does *not* mean we can all go
write lots of C++11 code yet. I at least want to let the dust settle
from the bots first.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202542 91177308-0d34-0410-b5e6-96231b3b80d8