Move storage for VFS into editor document

The invocation options are not an appropriate place to put this state,
since it can change between requests. This moves it to the editor
document, allowing us to change the specific VFS instance without
causing a rebuild (unless the contents/timestamps for a dependency
change).

tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp

@@ -220,7 +220,9 @@ void SwiftLangSupport::codeComplete(
     Optional<VFSOptions> vfsOptions) {
 
   std::string error;
-  auto fileSystem = getFileSystem(vfsOptions, error);
+  // FIXME: the use of None as primary file is to match the fact we do not read
+  // the document contents using the editor documents infrastructure.
+  auto fileSystem = getFileSystem(vfsOptions, /*primaryFile=*/None, error);
   if (!fileSystem)
     return SKConsumer.failed(error);