[llvm] [bazel] Fix lit tests with bazel 8.x (PR #119462)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

Ah, `legacy_external_runfiles` is mentioned briefly in https://github.com/bazelbuild/bazel/releases/tag/8.0.0.

Thanks for bisecting bazel to find what changed, and submit the fix!

https://github.com/llvm/llvm-project/pull/119462