Clang: Add clang query

Clang query is mechanism to use AST matcher to search for code. Think
about regular expression but in the context of AST. So you get a semantic
search tool for C++.

Change-Id: I72e882c5b53a0c52f352a3664847c4c3e4f6fc2e
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/clangrefactoring/refactoringengine.h

@@ -51,7 +51,6 @@ public:
 private:
     ClangBackEnd::RefactoringServerInterface &server;
     ClangBackEnd::RefactoringClientInterface &client;
-    bool isUsable_ = false;
 };
 
 } // namespace ClangRefactoring