[Sema] Fix a warning

This patch fixes:

  clang/lib/Sema/SemaTemplateVariadic.cpp:1069:22: error: variable
  'TST' set but not used [-Werror,-Wunused-but-set-variable]

Author: kazutakahirata

clang/lib/Sema/SemaTemplateVariadic.cpp

@@ -1071,6 +1071,7 @@ UnsignedOrNone Sema::getNumArgumentsInExpansionFromUnexpanded(
                        .first.dyn_cast<const TemplateSpecializationType *>()) {
       // This is a dependent pack, we are not ready to expand it yet.
       assert(isPackProducingBuiltinTemplateName(TST->getTemplateName()));
+      (void)TST;
       return std::nullopt;
     } else if (auto *PST =
                    Unexpanded[I]