Use typename on templates for consistency. (#6038)

They're essentially equivalent, we just typically write `typename`; even
in the examples here, most have other templates in the same file that
use `typename`.

common/map_test.cpp

@@ -17,7 +17,7 @@
 // Workaround for std::pair comparison deficiency in libc++ 16.
 #if defined(_LIBCPP_VERSION) && _LIBCPP_VERSION < 170000
 namespace std {
-template <class T, class U, class V, class W>
+template <typename T, typename U, typename V, typename W>
   requires(convertible_to<V, T> && convertible_to<W, U>)
 inline auto operator==(
     pair<std::reference_wrapper<T>, std::reference_wrapper<U>> lhs,