invoke_type members for free- and member-invoke traits containers

Summary:
[Folly] `invoke_type` members for free- and member-invoke traits containers.

Usable as:

```
FOLLY_CREATE_FREE_INVOKE_TRAITS(swap_invoke_traits, swap, std);
// ...
swap_invoke_traits::invoke_type my_swap;
my_swap(obj1, obj2);

Reviewed By: aary

Differential Revision: D8670799

fbshipit-source-id: 0c24912721aa20ca9647261e24ed3eb0e340ab20

folly/functional/Invoke.h

@@ -192,6 +192,8 @@ struct free_invoke_proxy {
       -> decltype(Invoke{}(static_cast<Args&&>(args)...)) {
     return Invoke{}(static_cast<Args&&>(args)...);
   }
+
+  using invoke_type = Invoke;
 };
 
 } // namespace detail
@@ -214,6 +216,10 @@ struct free_invoke_proxy {
  *
  *  * invoke
  *
+ *  And a member type alias:
+ *
+ *  * invoke_type
+ *
  *  These members have behavior matching the behavior of C++17's corresponding
  *  invocation traits types, aliases, and functions, but substituting canonical
  *  invocation with member invocation.
@@ -326,6 +332,8 @@ struct member_invoke_proxy {
       -> decltype(Invoke{}(static_cast<O&&>(o), static_cast<Args&&>(args)...)) {
     return Invoke{}(static_cast<O&&>(o), static_cast<Args&&>(args)...);
   }
+
+  using invoke_type = Invoke;
 };
 
 } // namespace detail
@@ -348,6 +356,10 @@ struct member_invoke_proxy {
  *
  *  * invoke
  *
+ *  And a member type alias:
+ *
+ *  * invoke_type
+ *
  *  These members have behavior matching the behavior of C++17's corresponding
  *  invocation traits types, aliases, and functions, but substituting canonical
  *  invocation with member invocation.