[llvm] Allow specifying libcxx builder image. (PR #110303)

[Michael Buch via llvm-commits]

Michael137 wrote:

https://github.com/llvm/llvm-project/pull/113593

If you want to make sure that this definitely resolves the issue @EricWF, you could cherry-pick that change and let the PR CI run again?

https://github.com/llvm/llvm-project/pull/110303