[llvm] [Bazel] Add support for Bzlmod (PR #88927)

[Aaron Siddhartha Mondal via llvm-commits]

aaronmondal wrote:

> Seems like the two issues are resolved now. Any chance you want to try the transition again?

Will retry this weekend :+1:

I'll need to figure out a more elegant solution for the patching and I think I figured out a better way for the external dependencies via label-flags so that we no longer need to maintain the custom nonhermeticity logic.

https://github.com/llvm/llvm-project/pull/88927