Fix "unused parameter" in getValueOrUnit in non-debug builds

Summary:
`value` could trigger "unused parameter" warning in NDEBUG builds. The fix is straightforward, to use the new-ish `[[maybe_unused]]` attribute.

The alternatives are to use `FOLLY_MAYBE_UNUSED` or even manual `(void)value;`. But since this is coroutines library, it should be reasonably safe to assume the compiler supports the cleaner and more expressive native C++17 option.

Reviewed By: lewissbaker

Differential Revision: D21784931

fbshipit-source-id: 895bf95d65df6533d768b65c44a898a73e0a636f

folly/experimental/coro/Collect-inl.h

@@ -30,7 +30,7 @@ T&& getValueOrUnit(Try<T>&& value) {
   return std::move(value).value();
 }
 
-inline Unit getValueOrUnit(Try<void>&& value) {
+inline Unit getValueOrUnit([[maybe_unused]] Try<void>&& value) {
   assert(value.hasValue());
   return Unit{};
 }