[llvm] [Bazel] Migrate to Bzlmod (PR #88927)

[Aaron Siddhartha Mondal via llvm-commits]

aaronmondal wrote:

Note: This is now rebased onto (and dependent on) https://github.com/llvm/llvm-project/pull/126729 to separate flag config changes from the bzlmod changes.

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