[llvm] [Bazel] Support opt-driver library, like clang-driver (PR #79205)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> This allows downstream users to make their own variations of clang, without having to redo/maintain separate build pipelines.

Consider naming the project, so if other people do any change, they can find the project name from the commit message...

It may be worth testing the following configurations:

```
default
-DBUILD_SHARED_LIBS=on
-DLLVM_LINK_LLVM_DYLIB=on
```

https://github.com/llvm/llvm-project/pull/79205