Auto merge of #118073 - saethlin:gc-dead-allocs, r=RalfJung

Miri: GC the dead_alloc_map too

dead_alloc_map is the last piece of state in the interpreter I can find that leaks. With this PR, all of the long-term memory growth I can find in Miri with programs that do things like run a big `loop {` or run property tests is attributable to some data structure properties in borrow tracking, and is _extremely_ slow.

My only gripe with the commit in this PR is that I don't have a new test for it. I'd like to have a regression test for this, but it would have to be statistical I think because the peak memory of a process that Linux reports is not exactly the same run-to-run. Which means it would have to not be very sensitive to slow leaks (some guesswork suggests for acceptable CI time we would be checking for like 10% memory growth over a minute or two, which is still pretty fast IMO).

Unless someone has a better idea for how to detect a regression, I think on balance I'm fine with manually keeping an eye on the memory use situation.

r? RalfJung

Author: bors