Fix zombie processes being left behind when resolving using atos

[s-ludwig]

The code for resolving debug information using atos, which is mainly used by LDC, as it doesn't embed line information into the executable (by default), never called waitpid() on the child PID, leading to zombie processes and an eventual exhaution of open file descriptors.

Fixes #21774.

[dlang-bot]

Thanks for your pull request, @s-ludwig!

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Testing this PR locally

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dub run digger -- build "stable + dmd#21775"