[clang] [clang] Be const-correct with all uses of `Module *`. (PR #93493)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> Errr, not certain I agree with this -- that basically is "admit defeat and stop aiming for const correctness."

Well, I am saying, add const to places we are pretty sure we will never change, and leave const out when in doubt.
Don't add const just because we don't need mutation today.

https://github.com/llvm/llvm-project/pull/93493