Add a missing return to the move assignment operator for

SequenceNumberManager.

Sadly, we don't have any unittests for this class because it is
a private class. Since it seems to have a nice isolated and testable
interface, it'd be great to extract it to a detail namespace and write
unit tests for it as then we could catch issues. I'll probably pester
Lang about that or some alternative refactoring.

This was noticed by PVS-Studio.

llvm-svn: 285990