Rename IDocument::fileName --> filePath

That is what it actually is, wrt how Qt API calls it.

Change-Id: Ied02055debf6aad75556b0d9d22e8ba2f72be555
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/cpptools/cppcompletionassist.cpp

@@ -440,7 +440,7 @@ public:
         return new CppTools::Internal::CppCompletionAssistInterface(
                     document,
                     position,
-                    editor()->document()->fileName(),
+                    editor()->document()->filePath(),
                     reason,
                     modelManager->snapshot(),
                     includePaths,