compilerplugins: getAsTagDecl doesn't exist in clang 3.5

Change-Id: If7277820227486e2eb578a167e0fd9f05c8f74ae
Reviewed-on: https://gerrit.libreoffice.org/19823
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

compilerplugins/clang/stringconstant.cxx

@@ -235,7 +235,7 @@ bool StringConstant::VisitCallExpr(CallExpr const * expr) {
                 && t->isClassType()
                 && t->getAs<SubstTemplateTypeParmType>() == nullptr)
             {
-                auto td = t->getAsTagDecl();
+                auto td = compat::getAsTagDecl(*t);
                 auto id = td->getIdentifier();
                 if (id != nullptr && id->isStr("OUString")) {
                     auto nd = dyn_cast<NamespaceDecl>(td->getParent());