[llvm] [bazel] Add support for LLVM_TOOL_LLVM_DRIVER_BUILD (PR #86879)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

Looks great! I don't know too much about Bazel to check the rules but the code looks reasonable. I just made some nits. 

https://github.com/llvm/llvm-project/pull/86879