avoid having to manuall modify sources when adding a new clang plugin

Now each one registers in its .cxx file.

Change-Id: I811c0d4400c2bdccc1c287269378d7e8ad8743ce

compilerplugins/clang/postfixincrementfix.hxx

@@ -22,7 +22,7 @@ class PostfixIncrementFix
     {
     public:
         explicit PostfixIncrementFix( ASTContext& context, Rewriter& rewriter );
-        void run();
+        virtual void run();
         bool VisitFunctionDecl( FunctionDecl* declaration );
     private:
         typedef std::vector< const Stmt* > StmtParents;