[CMake] Move configuration of LLVM_CXX_STD to HandleLLVMOptions.cmake

Standalone builds of projects other than llvm itself (lldb, libcxx,
etc) include HandleLLVMOptions but not the top level llvm CMakeLists,
so we need to set this variable here to ensure that it always has a
value.

This should fix the build issues some folks have been seeing.

llvm-svn: 357976