loplugin: use TypeCheck instead of getQualifiedNameAsString

since the latter is rather slow

Change-Id: Ib73cdb923585580777c2265b561c1808e93b2baa
Reviewed-on: https://gerrit.libreoffice.org/33585
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

compilerplugins/clang/stringconstant.cxx

@@ -1626,8 +1626,7 @@ void StringConstant::handleOUStringCtor(
     if (e3 == nullptr) {
         return;
     }
-    if (e3->getConstructor()->getQualifiedNameAsString()
-         != "rtl::OUString::OUString")
+    if (!loplugin::DeclCheck(e3->getConstructor()).Function("OUString").Class("OUString").Namespace("rtl").GlobalNamespace())
     {
         return;
     }