[feature request] Add LLDB

I see that LLDB is regularly built and shipped with prebuilt LLVM on GitHub.

Just like `@llvm_toolchain//:clang-format` exists, would it be possible to add `@llvm_toolchain//:lldb` so that LLDB could be used for debugging without the need to install one?

Would then something like `bazel run //:my-target --run_under=@llvm_toolchain//:lldb` even work?

Start by tracing how the existing @llvm_toolchain//:clang-format target is defined and how prebuilt LLVM artifacts are exposed. Check whether the proposed @llvm_toolchain//:lldb target can support the shown Bazel --run_under invocation; done means LLDB is exposed and that usage is verified.

Written by the indexing model from the issue text.