gmock-actions: make OnceAction public.

So that it can be referenced in conversion operators for actions that need to
know the concrete return type.

PiperOrigin-RevId: 447889344
Change-Id: I643d3298bc8effd08741282a956c221f9d67d378

googlemock/test/gmock-actions_test.cc

@@ -1920,7 +1920,7 @@ TEST(MockMethodTest, ActionSwallowsAllArguments) {
 
 struct ActionWithTemplatedConversionOperators {
   template <typename... Args>
-  operator internal::OnceAction<int(Args...)>() && {  // NOLINT
+  operator OnceAction<int(Args...)>() && {  // NOLINT
     return [] { return 17; };
   }