[llvm] t[Github] Add initial version of precommit checks (PR #80951)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

I think this is in a workable enough state at this point given the recent container work. After this (and fixing any following issues), I plan on working on the following:
* Enabling more subprojects using logic from the existing premerge checks to select what to build to test (as per the RFC discussion).
* Introducing a windows docker container/workflow to get testing coverage back there in hopefully a reasonably performant manner, potentially only running after Linux tests have passed to reduce job load.
* Potentially moving to label-triggered tests so we could potentially make this required at some point and could enable opt-in tests.

Probably a couple other misc things in there. This will help us figure out system scalability however and get things rolling in those directions.

https://github.com/llvm/llvm-project/pull/80951