Add a missing return to the move assignment operator for SequenceNumberManager.
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.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@285990 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm