Build LLVM distribution with our custom patches

[werat]

Some functionality of lldb-eval depends on the patches we contributed to LLDB -- https://github.com/google/lldb-eval/blob/master/docs/lldb-patches.md

Some of the patches are not yet submitted or not available in earlier versions of LLVM. For running pre-commit tests in GitHub Actions it would be nice to have these patches, so we can tests everything.