CppEditor: Reduce calls to recalculateSemanticInfo()

Case: Opening a file that is not yet in the Snapshot

recalculateSemanticInfoNow() emitted the same SemanticInfo until the
indexer processed the document (Snapshot::contains()). That signal
caused CppEditorWidget::updateSemanticInfo() to call
semanticRehighlight() because of unequal revisions. That triggered
recalculateSemanticInfo() again.

Change-Id: I8fb7d7c42aad3d6a89876e743cd4e3c7aff643bf
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/cpptoolseditorsupport.cpp

@@ -422,6 +422,8 @@ void CppEditorSupport::recalculateSemanticInfoNow(const SemanticInfo::Source &so
                 doc->control()->setTopLevelDeclarationProcessor(processor);
             doc->check();
             semanticInfo.doc = doc;
+        } else {
+            return;
         }
     }