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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@artagnon Do we need the `const ValueT &Default` variant?  I'm looking at `std::optional<T>::value_or` as a reference, and it only has the rvalue reference variant.

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