[CIR][cmake] Add support for cmake variable CLANG_ENABLE_CIR

Introduce a cmake variable that guards the inclusion of ClangIR into the
build of clang. Guard that we aren't trying to build without MLIR. Add
two subdirectories that, as of now, don't do anything.

Reviewers: bcardosolopes, erichkeane, petrhosek, Ericson2314

Reviewed By: bcardosolopes

Pull Request: https://github.com/llvm/llvm-project/pull/86078