stdlib: Fix the signature of _taskEscalate to return a priority

The `swift_task_escalate` is defined to return the new priority of the
task after the escalation but the silgen_name'd function did not have
the return type specified.

Follow up to 18c2584

Author: kateinoigakukun

stdlib/public/Concurrency/Task.swift

@@ -1494,7 +1494,8 @@ internal func _taskCurrentPriority(_ task: Builtin.NativeObject) -> UInt8
 
 @available(SwiftStdlib 6.2, *)
 @_silgen_name("swift_task_escalate")
-internal func _taskEscalate(_ task: Builtin.NativeObject, newPriority: UInt8)
+@discardableResult
+internal func _taskEscalate(_ task: Builtin.NativeObject, newPriority: UInt8) -> UInt8
 
 @available(SwiftStdlib 5.1, *)
 @_silgen_name("swift_task_basePriority")