[llvm] [bazel] Flip --enable_bzlmod to true (PR #168555)

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

I don't know what kind of testing we need to do for this  -- besides our own CI, which passes. How much time do people need to test this out in their own repos? Is it easier to just land this now and revert later if downstream users discover issues? Unless others have opinions, I'll plan on giving this a week.

https://github.com/llvm/llvm-project/pull/168555