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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

It is missing tests. I don't like the SB prefix. Could you instead put everything into `namespace llvm::sandboxir`. No worries if the conflict between llvm::Value and the SandBoxIR Value is too large.

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