[llvm] [SandboxIR] Boilerplate code (PR #95814)

[via llvm-commits]

vporpo wrote:

> Each commit/PR comes with a test.

I am aware of this rule, but there is no functionality to exercise at this point with a test, since this is just boilerplate code and all we really care about is that it builds and is registered correctly with CMake and the other build systems.
I am happy to add tests to the initial patch, but the patch will end up being a **lot** larger than this, as it would have to include a lot more code.

Regarding the namespace, I think the additional benefit of using a namespace is that one could potentially switch some code over from LLVM IR to Sandbox IR by replacing `using namespace llvm` to `using namespace sandboxir` (with some additional code changes of course). I am fine either way.

https://github.com/llvm/llvm-project/pull/95814