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

[Piotr Fusik via llvm-commits]

pfusik wrote:

When I see a non-const method, reference or pointer, I wonder how the object gets modified. Then there's an "aha" moment that it actually doesn't get modified and I add `const` to avoid repeating this process. I believe that's beneficial for other people looking at the code for the first time.

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