[BuildSystem] Guard killAfterTimeoutThread in LaneBasedExecutionQueue
[BuildSystem] Guard killAfterTimeoutThread in LaneBasedExecutionQueue

TSAN tests show a potential data race in cancellation. Add a guard
around the killAfterTimeoutThread variable to protect against it.

rdar://problem/40707314

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild