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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Could you add a Value.cpp to  llvm/unittests/Transforms/sandboxir/ with a small instantiation of sandboxir::Value to meet the test requirements?

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