[sourcekit] Synchronize cursor-info requests on generated interfaces

Cursor info requires access to the underlying AST, which is not
thread-safe. This manifest as crashes when performing concurrent
cursor-info requests on the same generated interface. We already
prevented concurrent cursor-infos on regular Swift files by using the
ASTManager, but generated interfaces use the InterfaceGenContext which
may use either an ASTUnit or its own internal CompilerInstance.

rdar://problem/27311624

tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp

@@ -209,6 +209,10 @@ namespace SourceKit {
   EditorDiagConsumer &ASTUnit::getEditorDiagConsumer() const {
     return Impl.CollectDiagConsumer;
   }
+
+  void ASTUnit::performAsync(std::function<void()> Fn) {
+    Impl.Queue.dispatch(std::move(Fn));
+  }
 }
 
 namespace {