Clang: Introduce DocumentParsed as job requirement

This is in prepration for a follow-up change, which introduces a state
where the Document is not yet parsed (it was reset) but the queue has
jobs.

All jobs get this requirement except those that generate an AST.

Change-Id: Ifcbb704e54108b40797180514c5ad3f3768ef10b
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangbackend/source/clangtranslationunits.h

@@ -65,6 +65,7 @@ public:
     void updateParseTimePoint(const Utf8String &translationUnitId, TimePoint timePoint);
 
     bool areAllTranslationUnitsParsed() const;
+    bool hasParsedTranslationUnit() const;
 
 public: // for tests
     int size() const;