[llvm] [CI][Github] Enable CIR CI build and test (PR #147430)

[Andy Kaylor via llvm-commits]

andykaylor wrote:

> > Triggering at least a build with CIR enabled when MLIR is changed is one of the main motivators of this PR. We've had a couple of instances recently where MLIR changes broke CIR. Currently the check-clang-cir tests don't take long at all -- less than 10 seconds for 170 tests. In the CIR incubator project, there are 570 tests, which still run in about 30 seconds.
> 
> I didn't realize they were that fast. That makes me a bit more comfortable with this. What sort of machine are you getting the 30 seconds figure from?

That was running on a single CPU AMD EPYC 7513 (Milan). I ran the tests (both CIR and clang codegen) with a debug build and all targets enabled, but otherwise I'm not sure why the clang codegen tests took so much longer for me.

> It's not just building clang with CIR enabled. Currently for MLIR changes we aren't building clang. After this patch we will be (and with CIR enabled).

Are you sure? Because even before this change flang has a dependency on clang in `compute_projects.py` and MLIR has flang as a "dependent to test".

https://github.com/llvm/llvm-project/pull/147430