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

[Ramkumar Ramachandra via llvm-commits]

artagnon 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.

Yes, there seems to be a bug in MSVC, and I _think_ creating a `const ValueT &` variant would fix it?

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