Use Github Actions

[rengolin]

We're looking into getting pre-merge checks from pull requests, but our CI runs on local machines. To avoid running arbitrary code locally, we don't allow PRs to trigger CI.

This can be solved with Github actions, at least for some basic testing (build + check), which should be enough for pre-merge. Like LLVM, if we have a problem that hasn't been found by pre-merge tests, we create an artificial IR-to-IR test, which is quick and runs well on Github builders.

There's an initial implementation on branch github-actions. The LLVM and sandbox builds work fine, the integration between them doesn't.

Anyone with a few days to spare and a weird passion for infrastructure jobs can pick where I left off. :)

@hfp

[rengolin]

We can use Github releases to store LLVM builds, in a similar way that LLVM itself does.

With an action to create and upload the release using github's own Python library.

The end result will look like this, and we can download with Wget on the compiler build.

[rengolin]

Our CI has been pretty stable, so I'll close this ticket. If we want to do this again, we can reopen.