[llvm] [SandboxVec] Boilerplate (PR #107431)

[via llvm-commits]

gulfemsavrun wrote:

We started seeing a `CMake` error after this patch.
```
CMake Error: install(EXPORT "LLVMExports" ...) includes target "LLVMVectorize" which requires target "LLVMSandboxIR" that is not in any export set.
```
https://luci-milo.appspot.com/ui/p/fuchsia/builders/prod/llvm-linux-arm64/b8737544888773099041/overview

https://github.com/llvm/llvm-project/pull/107431