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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

Gentle ping. The `const_arg_t<ValueT>` is incorrect when the value in the map is non-cost. This patch isn't a blocker for anything yet, but it would be nice to get this fixed.

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