filter out some of the AST in the plugins

by checking if the current namespace decl is in our code, so we have to
scan less stuff, which results in a 10% perf improvement for me

Change-Id: Idf0e30d57b6d0dcd13daa9ed679c28b9d233d387
Reviewed-on: https://gerrit.libreoffice.org/58942
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

compilerplugins/clang/unusedmethods.cxx

@@ -75,11 +75,11 @@ static std::set<MyFuncInfo> calledFromOutsideSet;
 
 
 class UnusedMethods:
-    public RecursiveASTVisitor<UnusedMethods>, public loplugin::Plugin
+    public loplugin::FilteringPlugin<UnusedMethods>
 {
 public:
     explicit UnusedMethods(loplugin::InstantiationData const & data):
-        Plugin(data) {}
+        FilteringPlugin(data) {}
 
     virtual void run() override
     {