Change result_of to invoke_result in fatal/container/legacy_varia…

…nt.h

Summary: C++20 has [eliminated](https://en.cppreference.com/w/cpp/types/result_of) `result_of` in favour of `invoke_result`. It's mysterious that this code even still works, but, nevertheless, I'm fixing it.

Differential Revision: D56987396

fbshipit-source-id: 71ca954c5825fb8f0e1939e51c77fe700b1dff20

fatal/container/legacy_variant.h

@@ -1283,7 +1283,7 @@ struct legacy_variant {
   template <typename UCallable, typename... UArgs, typename std::enable_if<
     is_supported<
       typename std::decay<
-        typename std::result_of<UCallable&(UArgs...)>::type
+        typename std::invoke_result<UCallable&, UArgs...>::type
       >::type
     >(), int
   >::type = 0>
@@ -1295,7 +1295,7 @@ struct legacy_variant {
   template <typename UCallable, typename... UArgs, typename std::enable_if<
     !is_supported<
       typename std::decay<
-        typename std::result_of<UCallable&(UArgs...)>::type
+        typename std::invoke_result<UCallable&, UArgs...>::type
       >::type
     >(), int
   >::type = 0>