`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.
We previously waited for the initialization response from the build server during the creation of a `Workspace` so that we could create a `SemanticIndexManager` with the index store path etc. that was returned by the `build/initialize` response. This caused all functionality (including syntactic) of SourceKit-LSP to be blocked until the build server was initialized.
Change the computation of the `SemanticIndexManager` and related types to happen in the background so that we can provide functionality that doesn’t rely on the build server immediately.
Fixes#2304
