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

[Louis Dionne via llvm-commits]

ldionne wrote:

@EricWF The CI failures are unrelated issues on `main` that have been fixed. It looks like everything is working now.

I'll let you merge this since you likely want to adjust the capacity and other stuff before or closely after you merge, but as far as I'm concerned this is good to go. Thanks a whole lot for this improvement!

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