Clang: Fix updating supportive translation unit

...with new project arguments.

Reproduce with:
 1. Open a file with some #ifdef
 2. Edit the file, e.g. type space.
 3. Change the build configuration so that the highlighting flips in the
    editor.
 4. Edit the file again, the configuration flips again [repeat].

Change-Id: Ib97c18fc46538d74c63972e682a348ae2736a4e6
Reviewed-by: David Schulz <david.schulz@qt.io>

src/tools/clangbackend/ipcsource/clangdocument.h

@@ -102,7 +102,7 @@ public:
 
     bool isDirty() const;
     TimePoint isDirtyTimeChangePoint() const;
-    void setDirtyIfProjectPartIsOutdated();
+    bool setDirtyIfProjectPartIsOutdated();
     void setDirtyIfDependencyIsMet(const Utf8String &filePath);
 
     TranslationUnitUpdateInput createUpdateInput() const;