base/TimerManager: Clamp timer expiration time to maxBound

Previously we would allow the expiration time to overflow, which in
practice meant that `threadDelay maxBound` we return far earlier than
circa 2500 CE. For now we fix this by simply clamping to maxBound.

Fixes #15158.

Test Plan: Validate, run T8089

Reviewers: simonmar, hvr

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15158

Differential Revision: https://phabricator.haskell.org/D4719