[llvm] [lit] Add support for readfile to external shell (PR #159431)

[James Henderson via llvm-commits]

jh7370 wrote:

That looks suspiciously like a buffered output not being flushed properly somewhere, perhaps being flushed in some racy manner somewhere within the system implementation. I don't really have a concrete suggestion as to where though, nor can I be certain that is the issue.

https://github.com/llvm/llvm-project/pull/159431