[llvm] [SandboxIR][NFC] Move Value class into a separate file (PR #110059)

[via llvm-commits]

Sterling-Augustine wrote:

Buildbot failure is real. Probably needs some forward declarations for MSVC like those at the top of Type.h.

Once the build issues are resolved, looks good.

https://github.com/llvm/llvm-project/pull/110059