Auto merge of #134438 - lqd:const-qualif-bitsets, r=compiler-errors

Use `MixedBitSet`s in const qualif

These analyses' domains should be very homogeneous, having compressed bitmaps on huge cfgs should make a difference (and doesn’t have an impact on the smaller / regular cfgs in our benchmarks).

This is a >40% walltime reduction on [this stress test](https://github.com/Manishearth/icu4x_compile_sample) extracted from a real world ICU case, and a 10x or so max-rss reduction.

cc `@oli-obk` `@RalfJung`

Should help with (or fix) issue #134404.

Author: bors