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

[David Stone via cfe-commits]

davidstone wrote:

> Can you make sure that at every place this PR touches `const` makes sense? I found out recently that we can be quite good at pretending that something is `const`, all the way down until we realize we need a `const_cast`, because modification is required in that one place.

To add a little more flavor to my response, this PR is a precursor to a larger refactoring of `Module` itself. I plan on making some changes to it that require even less mutation for use (in other words, there are places in the code that I could not mark `const` in this PR but in the future we will be able to). I wanted to get all the trivial `const` stuff correct now so that my future changes will be smaller.

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