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

[Andy Kaylor via llvm-commits]

andykaylor wrote:

> I didn't quite get what you mean by "that" here?

Sorry. I meant having a different set of tests run for testing as a test dependency than are run when the project itself is modified. For instance, when CIR files are modified we want to test the "check-clang-cir" target, but when CIR testing is being run because the MLIR project was modified maybe we want to test something like "check-clang-cir-min". My question was whether there would be other cases where we'd want to run a subset of tests in the dependency case.

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