chore: add workflow to keep in sync with upstream

[void2012]

Hi! Really appreciate your works. But this fork is a little bit outdated, so could you please update it to the newest version of LLVM?

[widberg]

Sorry, I've been busy with a lot recently. I plan on getting back into this soon. As part of updating to the latest LLVM, I want to make some changes to how I'm maintaining this project to hopefully make it easier going forward. I'll lay out these ideas here and use this issue to track the progress.

• [x] Maintain a patch set using something like lappverk or quilt instead of rebasing commits on LLVM main.
• [x] Make sure my main branch has the patch set applied to it so Compiler Explorer keeps working.
  • [x] Ask Compiler Explorer folks if they prefer this or having a way to apply the patches without cloning a second repository.
• [ ] Regularly, weekly/monthly, apply the patch set to the latest LLVM and run the tests in GitHub Actions.
• [ ] Automatically update a nightly release if the Action succeeds, notify me, and fix it if it fails.
  • [ ] Do the same thing for each new LLVM tag.
• [ ] Figure out a way to use something like ccache in Actions so this stuff runs quickly.

Hopefully, that can eliminate some of the maintenance burden on me. A lot of the reason I avoid touching this project is because of how hard I made it to modify, which is exacerbated by how quickly LLVM updates break my patches. If you have any thoughts as a consumer of the compiler, I'd love to hear them.

[widberg]

I made a new release based on llvmorg-21.1.0. I'll work on resolving the remaining points before closing this.