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

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

This PR needs to update the tests in `compute_projects_tests.py` and potentially add new ones if relevant.

Do you know how much additional time it takes to test ClangIR, particularly on changes to MLIR? MLIR changes are frequent and adding a clang build/checks has the potential to significantly increase the overall premerge time.

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