[CMake] Replace LLVM_ENABLE_CXX1Y and friends with LLVM_CXX_STD
Simplify building with particular C++ standards by replacing the
specific "enable standard X" flags with a flag that allows specifying
the standard you want directly.
We preserve compatibility with the existing flags so that anyone with
those flags in existing caches won't break mysteriously.
Differential Revision: https://reviews.llvm.org/D60399