Extirpate FOLLY_CONSTEVAL from folly/futures/Promise.h

Summary: All code should be on C++20 now.

Reviewed By: yfeldblum

Differential Revision: D88981912

fbshipit-source-id: aacfea08e0f3d15abce6b370122a0bc7631fa8fe

Author: r-barnes

third-party/folly/src/folly/futures/Promise.h

@@ -58,7 +58,7 @@ class FOLLY_EXPORT FutureAlreadyRetrieved : public PromiseException {
 class FOLLY_EXPORT BrokenPromise : public PromiseException {
  private:
   template <typename T>
-  static FOLLY_CONSTEVAL auto make_error_message() {
+  static consteval auto make_error_message() {
     constexpr auto prefix =
         detail::pretty_carray_from("Broken promise for type name `");
     constexpr auto prefix_size = std::size(prefix.data);