Clang: implement findUsages with existing index

Functionality is limited to the abilities of
current index which is not updated and is
generated only at project open.
Search box temporarily doesn't allow to "Search again".

Change-Id: Id1047f27ad0aafc901f06aa51ad38ceab95eaebb
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/refactoringengineinterface.h

@@ -27,8 +27,10 @@
 
 #include "cpptools_global.h"
 #include "cursorineditor.h"
+#include "usages.h"
 
 #include <utils/fileutils.h>
+#include <utils/smallstring.h>
 
 #include <clangsupport/sourcelocationscontainer.h>
 #include <clangsupport/refactoringclientinterface.h>
@@ -53,10 +55,14 @@ class CPPTOOLS_EXPORT RefactoringEngineInterface
 public:
     using RenameCallback = ClangBackEnd::RefactoringClientInterface::RenameCallback;
 
+    virtual ~RefactoringEngineInterface() {}
+
     virtual void startLocalRenaming(const CursorInEditor &data,
                                     CppTools::ProjectPart *projectPart,
                                     RenameCallback &&renameSymbolsCallback) = 0;
     virtual void startGlobalRenaming(const CursorInEditor &data) = 0;
+    virtual void findUsages(const CppTools::CursorInEditor &data,
+                            UsagesCallback &&showUsagesCallback) const = 0;
     virtual bool isRefactoringEngineAvailable() const { return true; }
 };