In the fallback build system use the URIs pseudo path to construct
compiler arguments. For non-file URIs this means passing the raw URI as
the filename. We were already testing non-existing file-URLs like /a.txt
and arbitrary URIs seem to allow semantic functionality as well.
According to the LSP specification, arbitrary URIs can be used as
document identifiers. Instead of internally assuming that all URIs are
URLs, use a DocumentURI enum to represent URIs. These can either be file
URLs or other URIs whose value as treated as an opaque string.
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
