[llvm] [lit] Echo full RUN lines in case of external shells (PR #66408)

[via llvm-commits]

dyung wrote:

> I think we're past that problem now. The remaining failure appears to be a `/` vs. `\` issue in a new test introduced by [D150987](https://reviews.llvm.org/D150987). (It also happens to be related to `/tmp`, but I think that's a coincidence.)

Yeah, I think so as well. I'm testing a fix for that issue.

https://github.com/llvm/llvm-project/pull/66408