Change result_of to invoke_result in mcrouter/McrouterFiberContext.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: D56987436

fbshipit-source-id: 73c86ace2ef88a26d9f8daa1ea5cdf42d749fea4

mcrouter/McrouterFiberContext.h

@@ -119,7 +119,7 @@ class fiber_local {
    * Clear all locals, run `f`, restore locals
    */
   template <class F>
-  static typename std::result_of<F()>::type runWithoutLocals(F&& f) {
+  static typename std::invoke_result<F>::type runWithoutLocals(F&& f) {
     auto guard = makeGuardHelperReset();
 
     return f();
@@ -129,7 +129,7 @@ class fiber_local {
    * Copy all locals, run `f`, restore locals
    */
   template <class F>
-  static typename std::result_of<F()>::type runWithLocals(F&& f) {
+  static typename std::invoke_result<F>::type runWithLocals(F&& f) {
     auto guard = makeGuardHelperCopy();
 
     return f();