Clang: Rename ModelManagerSupportClang::instance to instance_forTestsOnly

To make it clear that it is for testing only.

Change-Id: If257b0049e87c834b24073991f173bfc1b3e93e7
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/plugins/clangcodemodel/clangmodelmanagersupport.h

@@ -60,8 +60,9 @@ public:
 
     IpcCommunicator &ipcCommunicator();
 
-public: // for tests
-    static ModelManagerSupportClang *instance();
+#ifdef QT_TESTLIB_LIB
+    static ModelManagerSupportClang *instance_forTestsOnly();
+#endif
 
 private:
     void onEditorOpened(Core::IEditor *editor);