[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

llvm-svn: 357899