[llvm-branch-commits] [CIR][cmake] Add support for cmake variable CLANG_ENABLE_CIR (PR #86078)

[Nathan Lanza via llvm-branch-commits]

https://github.com/lanza created https://github.com/llvm/llvm-project/pull/86078

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.