[CIR][NFC] Add scaffolding for the CIR dialect and CIROps.td

This adds no real content, it just incrementally adds some scaffolding
necessary to enable a future patch to just add our first few ops.

Test Plan:
```
$ cmake -Sllvm -Bbuild -DCLANG_ENABLE_CIR=1 \
  -DLLVM_ENABLE_PROJECTS='clang;mlir' \
  -DCMAKE_BUILD_TYPE=Release -GNinja
$ ninja -C build check-clang
$ ninja -C build MLIRCIROpsIncGen
$ ninja -C build MLIRCIR
```

Reviewers: AaronBallman, erichkeane, bcardosolopes

Reviewed By: erichkeane, AaronBallman, bcardosolopes

Pull Request: https://github.com/llvm/llvm-project/pull/86080