[llvm] [bazel] Add a bazel flag to enable building MLIR with CUDA support (PR #88856)

[Christian Sigg via llvm-commits]

chsigg wrote:

> I'd expect that if someone enables this flag then it's on them to provide all the tools necessary.

I would expect that people will use a flag if it's there. I'm not saying we shouldn't land this as is and in the current state it looks clean. I was just trying to understand what is already supported. E.g., it doesn't trigger any new tests that would require a CUDA toolkit to be found.

I would expect though that proper CUDA support with bazel would pull the toolkit as an external dependency. But that's clearly a separate next step.

https://github.com/llvm/llvm-project/pull/88856