[llvm] [NFC] Add `const` (PR #116611)

[Piotr Fusik via llvm-commits]

pfusik wrote:

> What's the motivation here? This seems like an odd collection of files.

That's just a minor refactoring I did while studying the code. `const` helps me understand what is modified and what is not - without the `const` I'm looking for how the value might change. It's probably obvious if you're familiar with the code, but not if you're new to it.
Do you think the code is worse after my change or I should split it per module?

https://github.com/llvm/llvm-project/pull/116611