C++: add env. var. to disable decl/def link tracking.

When the environment variable QTC_NO_FUNCTION_DECL_DEF_LINK_TRACKING is
set, no method declaration/definition tracking is performed. This is to
see if QTCREATORBUG-11262 is triggered due to the tracking.

Task-number: QTCREATORBUG-11262
Change-Id: Id4cb1fabf9304f8bf42393cb65c402adf8818e2a
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cppeditor/cppeditor.cpp

@@ -1031,6 +1031,10 @@ void CPPEditorWidget::updateFunctionDeclDefLink()
 
 void CPPEditorWidget::updateFunctionDeclDefLinkNow()
 {
+    static bool noTracking = qgetenv("QTC_NO_FUNCTION_DECL_DEF_LINK_TRACKING").trimmed() == "1";
+    if (noTracking)
+        return;
+
     if (Core::EditorManager::currentEditor() != editor())
         return;
     const Snapshot semanticSnapshot = d->m_lastSemanticInfo.snapshot;