[swift] Get correct ExitStatus if swift tasks receives signal (#581)

When swift tasks failed with signal, make sure the build system tag that
as a signal/exception instead of a regular exit with non-zero error
code.

Author: cachemeifyoucan

Sources/SWBTaskExecution/TaskActions/SwiftDriverJobTaskAction.swift

@@ -469,14 +469,18 @@ public final class SwiftDriverJobTaskAction: TaskAction, BuildValueValidatingTas
                 }
 
                 func processFinished(result: CommandExtendedResult) {
+                    guard let status = Processes.ExitStatus.init(rawValue: result.exitStatus) else {
+                        // nil means the job is stopped or continued. It should not call finished.
+                        return
+                    }
                     // This may be updated by commandStarted in the case of certain failures,
                     // so only update the exit status in output delegate if it is nil.
                     if outputDelegate.result == nil {
                         outputDelegate.updateResult(TaskResult(result))
                     }
                     self._commandResult = result.result
                     do {
-                        try plannedBuild?.jobFinished(job: driverJob, arguments: arguments, pid: pid.pid, environment: environment, exitStatus: .exit(result.exitStatus), output: output)
+                        try plannedBuild?.jobFinished(job: driverJob, arguments: arguments, pid: pid.pid, environment: environment, exitStatus: status, output: output)
                     } catch {
                         executionError = error.localizedDescription
                     }