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

[Prasoon Kumar via llvm-commits]

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prasoon054 wrote:

I used google antigravity for generating comments in some places. Seems like it didn't stick to my instructions about following the guidelines at https://google.github.io/styleguide/pyguide.html#38-comments-and-docstrings in most places. I'll fix all of those. Going forward I'll be more careful applying changes suggested by these models.

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