Speed up data dehydration (#79209)

Speed up data dehydration (#79209)

Now that we switched everything we wanted to switch to dehydrated data, I redid startup measurements.

I used a 30 MB app that doesn't actually do anything useful with those megabytes and just exits (to get a lot of dehydrated data into the startup path).

On Windows:

(The measurements were frustratingly noisy within a range that spans 10 ms.)

Without dehydration, the app exits in ~27 ms.
With dehydration, the app exits in ~37 ms.
With the fixes in this PR and dehydration, it exits in ~34 ms.

On Linux, the difference between not dehydrated and dehydrated is ~8 ms -> ~12 ms. I didn't re-measure with this PR. Possibly shaved off a millisecond.