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

[Keith Smiley via llvm-commits]

keith wrote:

I think we actually need to change this PR to mostly avoid one off http_archive calls. The problem is before if the archives were named the same thing they would be shared, but that is no longer true with bzlmod, so for a project like ours that pulls in llvm with bazel, we would end up with 2 copies of nanobind for example, since we're providing one, and this would provide a new one.

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. 

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