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

[Louis Dionne via llvm-commits]

ldionne wrote:

This is amazing!

About the tests, I am not certain why the transitive includes test started failing, but I ran into something similar in https://github.com/llvm/llvm-project/pull/109720. I think this may be how we're running `awk` or something like that (I wasn't able to reproduce).

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