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

[Prasoon Kumar via llvm-commits]

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prasoon054 wrote:

The block's primary job is appending the in-process stage to `procs`, and the parallel `proc_not_counts` / `proc_not_fail_if_crash` entries. Updating `default_stdin` is sort of tail effect for the next iteration.

> it probably makes sense to sequester that logic out of line some place, just to make it easier to reason about in isolation.

Makes sense. I'll pull it out and group it with other stage-construction helpers.

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