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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Is it only me that see utils/lit/tests/shtest-readfile-external.py failing randomly every now and then?
I don't know what's happening but there seem to be something flaky about this test because in downstream build bots we sometimes see it failing and then passing again on the same commit.

Yeah, I've seen this too in our builders. I'm of the same opinion as James that it seems like some buffering issue. I haven't had time to investigate. I'll hopefully get to it later today.

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