`IndexStoreDB` moves its index to the `saved` directory when it is deallocated. Because `IndexStoreDB` is primarily owned by `UncheckedIndex`, we rely on deallocating this object to save the index store. This is fairly brittle because various parts of the codebase may hold transient references to that object as reported in https://github.com/swiftlang/sourcekit-lsp/issues/2455#issuecomment-3873561003.
Explicitly remove the reference from `UncheckedIndex` to `IndexStoreDB`. While this still isn’t perfect because other parts of the code base may hold references to `IndexStoreDB` but those should be a lot rarer, resulting in a more consistent closing of the index.
