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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

I looked through the review and I think this is generally an improvement. We may find a need to relax some `const` later, but I did not spot any obvious concerns.

That said, I'd like @ChuanqiXu9 to give the final sign-off on this as modules code owner.

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