Remove `settings(for:)` from the BuildSystem API in favor of the change callback
- With the `settings(for:)` method removed, we instead block on a
callback from the `BuildSystemManager` which itself handles
the primary build system callback in conjunction with the fallback
system
- We queue up notifications / requests that reference documents
until we receive the callback.
- Remove language from FileBuildSettings as it is no longer needed,
Swift-specific working dir fixes are now in SwiftCompileCommand
- registerForChangeNotifications should inform its delegate
asynchronously instead of immediately
When testing the sourcekit-lsp binary, it is handy to be able to force
requests to be handled synchronously. This only affects the protocol
layer, not the implementation. This option is hidden from the help text
and should only be used for testing/debugging.
