[analyzer] Make directly bounded LazyCompoundVal as lazily copied

Previously, `LazyCompoundVal` bindings to subregions referred by
`LazyCopoundVals`, were not marked as //lazily copied//.

This change returns `LazyCompoundVals` from `getInterestingValues()`,
so their regions can be marked as //lazily copied// in `RemoveDeadBindingsWorker::VisitBinding()`.

Depends on D134947

Authored by: Tomasz Kamiński <tomasz.kamiński@sonarsource.com>

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D135136