[llvm] Reland "[bazel][mlir][python] Port #155741: stub auto-generation" (PR #159940)

[Alex Trotta via llvm-commits]

Ahajha wrote:

All that's left is the `WORKSPACE`, indifferent if this gets merged now (we don't use it), but someone else might want it.

https://github.com/llvm/llvm-project/pull/159940