Summary:
[Folly] Make `EventBase` enqueue `noexcept`.

It cannot really fail anyway in correct usage besides allocation failure, unless in the `EventBase` destructor and while draining and the `AlwaysEnqueue` variant is called.

Theoretically if a caller attempts to enqueue concurrently with `EventBase` dtor while in `consumeUntilDrained`, but either *not* in the `EventBase` thread or in the `EventBase` thread and using the `AlwaysEnqueue` variant, there is a race which can lead to termination.

Reviewed By: andriigrynenko

Differential Revision: D14114678

fbshipit-source-id: 9a0128d207f86ca34eb8a1d417766c095ed5e137