Clang: Fix sending (un)register translation unit

Handle the following use cases:

  1. Editor is opened
    --> Register file at backend.

  2. Close editor for which a project part exists
    --> Unregister file with project part.

  3. Close editor for which no project part exists (anymore)
    --> Unregister file with empty/default project part.

Change-Id: I2b253004a920cccef989876538ab552eacf6b45c
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>

src/plugins/cpptools/baseeditordocumentparser.h

@@ -47,6 +47,7 @@ public:
     static BaseEditorDocumentParser *get(const QString &filePath);
 
     struct Configuration {
+        bool stickToPreviousProjectPart = true;
         bool usePrecompiledHeaders = false;
         QByteArray editorDefines;
         ProjectPart::Ptr manuallySetProjectPart;