change some clang plugins to FilteringPlugins

As far as I can tell these do not need to scan everything, so the filtering
(and faster) base plugin is sufficient.

Change-Id: I0912a730bb5ec9670d1faa5c83f573902118cc95
Reviewed-on: https://gerrit.libreoffice.org/68872
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

compilerplugins/clang/stringstatic.cxx

@@ -22,13 +22,12 @@
 namespace {
 
 class StringStatic
-    : public clang::RecursiveASTVisitor<StringStatic>
-    , public loplugin::Plugin
+    : public loplugin::FilteringPlugin<StringStatic>
 {
 
 public:
     explicit StringStatic(loplugin::InstantiationData const& rData):
-        Plugin(rData) {}
+        FilteringPlugin(rData) {}
 
     void run() override;
     bool VisitVarDecl(VarDecl const*);