Properly pick up new events when reusing a muxnote
Properly pick up new events when reusing a muxnote

Changes for SR-5759 introduced a bug where disarmed events got dropped when
a muxnote is reused, due to erroneously setting dmn->dmn_events to the
combination of new events and currently armed events.

Combining existing events with the combination of new events and armed events
implements the intended behavior.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch