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

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Given the discussion in the area team meeting we can move ahead with this. Thanks for the discussion and sorry for the bit of extra process. I want to make sure these sorts of decisions are being made by the community and not just me (or Google).

Added a couple of inline comments.

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