new solver: remove provisional cache
The provisional cache is a performance optimization if there are large, interleaving cycles. Such cycles generally do not exist. It is incredibly complex and unsound in all trait solvers which have one: the old solver, chalk, and the new solver ([link](https://github.com/rust-lang/rust/blob/master/tests/ui/traits/new-solver/cycles/inductive-not-on-stack.rs)).
Given the assumption that it is not perf-critical and also incredibly complex, remove it from the new solver, only checking whether a goal is on the stack. While writing this, I uncovered two additional soundness bugs, see the inline comments for them.
r? `@compiler-errors`
