Introduce a `BuildSystemDelegate` to handle notifications from the build system
* `SourceKitServer` is the main delegate to process these notifications
* Currently limited to changes in `FileBuildSettings`
* Delegate informs the `BuildSystem` of files to watch via `registerChangeWatching(for: URL)` and `unregisterChangeWatching(for: URL)`
* In the future we could have more integration for handling changes in dependencies
Handling changes in `FileBuildSettings`
* `SourceKitServer` sends notifications to the internal LSPs informing them of any opened documents that have changes in their compiler flags
* For clangd, we send a notification to update the compilation database
* For SourceKit/sourcekitd we must close and reopen the file to force a new AST with the new compiler flags
1) Toolchain ID is no longer unique; XcodeDefault toolchains may be
registered multiple times as long as their path is different
(e.g. different Xcode versions). At the moment no other toolchain
may be duplicated.
2) `BuildSystem` specifies a `Toolchain` directly instead of a
Toolchain identifier.
3) New methods to access Toolchains in the registry:
- All toolchain(s) with the given identifier
- Toolchain (if any) for the given path
