Add support for heap profiling while using the nonmoving collector.

We greatly simply the implementation by disabling concurrent collection for
GCs when heap profiling is enabled. This entails that the marked objects on
the nonmoving heap are exactly the live objects.

Note that we match the behaviour for live bytes accounting by taking the size
of objects on the nonmoving heap to be that of the segment's block
rather than the object itself.

Resolves #22221