[CMake] Replace LLVM_ENABLE_CXX1Y and friends with LLVM_CXX_STD

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357899 91177308-0d34-0410-b5e6-96231b3b80d8