[llvm] [DenseMap] Fix constness issues with lookup_or (PR #139247)

[Aaron Puchert via llvm-commits]

aaronpuchert wrote:

Side note: I think that MSVC is incorrect and the reference initialization should be allowed according to [[dcl.init.ref]p5.4.2](https://eel.is/c++draft/dcl.init.ref#5.4.2), because `const SCEV*` and `const SCEVUnknown*` are not reference-related.

https://github.com/llvm/llvm-project/pull/139247