[llvm] [lit] Run builtin cat / diff in-process instead of spawning (PR #208024)

[Prasoon Kumar via llvm-commits]

prasoon054 wrote:

> @prasoon054 have you looked at the windows failure? it seems related to this change.
Yes. I looked at it. It's a dormant bug from my previous change to `diff.py`. I'll add a fix in this PR.

https://github.com/llvm/llvm-project/pull/208024