Use `FxIndexSet` to avoid sorting fake borrows
This fixes#96449, but I haven't yet been able to
make the reproducer work using `#[cfg]` attributes,
so we can't use the 'revision' infra to write a test
The previous implementation relied on sorting by `PlaceRef`.
This requires sorting by a `DefId`, which uses untracked state
(see #93315)
b862b438db