Auto merge of #14995 - servo:codegen-units, r=mbrubeck

Enable codegen-units=4 in debug mode

We previously had this and removed it because it caused runtime slowdowns
(discussion in #11102), but that was in release mode.

Debug mode is already very slow at runtime.
In my opinion, debug mode is mostly for the edit-compile-test cycle
which is currently dominated by compile times, so sacrificing runtime for compile time makes sense.

I’ve observed a ~30% improvement of compile times with this. This figure is consistent between a very fast desktop with 4 cores / 8 threads and a not-as-fast laptop with 2 cores / 4 threads.

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