Fix build on Windows: put ExitStatus.signalled under os check

Author: egorzhdan

Sources/SwiftDriver/Driver/ToolExecutionDelegate.swift

@@ -77,10 +77,12 @@ public struct ToolExecutionDelegate: JobExecutionDelegate {
         let finishedMessage = FinishedMessage(exitStatus: Int(code), pid: pid, output: output)
         message = ParsableMessage(name: job.kind.rawValue, kind: .finished(finishedMessage))
 
+#if !os(Windows)
       case .signalled(let signal):
         let errorMessage = strsignal(signal).map { String(cString: $0) } ?? ""
         let signalledMessage = SignalledMessage(pid: pid, output: output, errorMessage: errorMessage, signal: Int(signal))
         message = ParsableMessage(name: job.kind.rawValue, kind: .signalled(signalledMessage))
+#endif
       }
       emit(message)
     }

Sources/SwiftDriver/Execution/DriverExecutor.swift

@@ -55,8 +55,10 @@ extension DriverExecutor {
       switch result.exitStatus {
       case .terminated(let code):
         returnCode = Int(code)
+#if !os(Windows)
       case .signalled(let signal):
         returnCode = Int(signal)
+#endif
       }
       throw JobExecutionError.jobFailedWithNonzeroExitCode(returnCode, try result.utf8stderrOutput())
     }

Sources/SwiftDriver/Execution/MultiJobExecutor.swift

@@ -358,8 +358,10 @@ class ExecuteJobRule: LLBuildRule {
           if !job.kind.isCompile || code != EXIT_FAILURE {
             context.diagnosticsEngine.emit(.error_command_failed(kind: job.kind, code: code))
           }
+#if !os(Windows)
         case let .signalled(signal):
           context.diagnosticsEngine.emit(.error_command_signalled(kind: job.kind, signal: signal))
+#endif
         }
       }