gmock-spec-builders: support mocking const-qualified function types.

In particular this automatically gives us support for examples like the
following:

    using SomeFn = absl::AnyInvocable<R(Args...) const>;
    MockFunction<SomeFn> some_fn;

PiperOrigin-RevId: 921303527
Change-Id: I19bf59671781e85db65cc20c0d6ea10b056c528a

googlemock/include/gmock/gmock-spec-builders.h

@@ -1957,6 +1957,11 @@ struct SignatureOf<R(Args...)> {
   using type = R(Args...);
 };
 
+template <typename R, typename... Args>
+struct SignatureOf<R(Args...) const> {
+  using type = R(Args...);
+};
+
 template <template <typename> class C, typename F>
 struct SignatureOf<C<F>,
                    typename std::enable_if<std::is_function<F>::value>::type>