Allow discarding startOnMainActor results

Adding discardableResult to startOnMainActor

Author: etcwilde

stdlib/public/Concurrency/Task.swift

@@ -192,6 +192,7 @@ extension Task where Failure == Error {
     @_spi(MainActorUtilities)
     @MainActor
     @available(SwiftStdlib 5.9, *)
+    @discardableResult
     public static func startOnMainActor(
         priority: TaskPriority? = nil,
         @_inheritActorContext @_implicitSelfCapture _ work: __owned @Sendable @escaping @MainActor() async throws -> Success
@@ -213,6 +214,7 @@ extension Task where Failure == Never {
     @_spi(MainActorUtilities)
     @MainActor
     @available(SwiftStdlib 5.9, *)
+    @discardableResult
     public static func startOnMainActor(
         priority: TaskPriority? = nil,
         @_inheritActorContext @_implicitSelfCapture _ work: __owned @Sendable @escaping @MainActor() async -> Success