[llvm] [bazel] Add MODULE.bazel (PR #164891)

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

> Ideally we could move every one of these calls into the BCR which would solve these potential problems. Alternatively it is possible that we could move these calls into a separate module_extension and potentially share them that way.

Having them all be part of BCR sounds like a good end goal. I like the idea of using a module extension for now, just to make this a straightforward migration to bzlmod, and then we can work on migrating deps individually after that.

> I got a clean build with the following changes on top of this PR:

Thanks @nicholasjng! I will give that a try once this is in.

> IIRC the tricky part was to not break the "other" workflows that are not just the local builds. I think it was something where if you pull in the bazel deps and there is a MODULE.bazel file in it already, that it broke depending on llvm as external repo.

I included the fix you pointed to, but I haven't tested it out yet to confirm it works when using LLVM as an external repo in another project.

https://github.com/llvm/llvm-project/pull/164891