Instead of storing build settings inside the language servers based on update notifications from the build system, always call into the `BuildSystemManager` to get the build settings.
Overall, I think this is a much clearer separation of concerns and will allow us to remove `SourceKitServer.documentToPendingQueue` in a follow-up commit as `SwiftLanguageServer` can always directly call into `BuildSystemManager` to get build settings and we don’t need to wait for the initial notification to receive the first build settings.
This requies `BuildServerBuildSystem` to keep track of the build settings it has received from the BSP server.
`ClangLanguageServer` still caches build settings locally. `ClangLanguageServer` will change to the same pull-based model in a follow-up commit.
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