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

[Aaron Siddhartha Mondal via llvm-commits]

aaronmondal wrote:

Note to self:

- Make all the other deps from the workspace usable with the module build:
    - [ ] rules_foreign_cc
    - [ ] pybind11
    - [ ] apple support
    - [ ] gmp
    - [ ] mpfr
    - [ ] pfm
    - [ ] pybind11
    - [ ] robin_map
    - [ ] nanobind

- Literally no bazel user will care about stable llvm tags. Test registry override logic and add docs and a migration guide so that we have first-class support for pinning this module to custom commits.

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