[6.2]: Add platform executor module for FreeBSD

Swift concurrency defines a default platform executor. This was not
defined for FreeBSD resulting in build failures. Defining it to use the
Dispatch executor.

Scope: This only impacts FreeBSD.
Risk: Low. This change only affects FreeBSD.
Reviewers: @compnerd, @ktoso, @al45tair
Testing: Local testing to ensure Concurrency builds on FreeBSD.
Fixes: rdar://150643436

(cherry picked from commit 4a41e50)

Author: etcwilde

Runtimes/Core/Concurrency/CMakeLists.txt

@@ -81,6 +81,7 @@ add_library(swift_Concurrency
   PartialAsyncTask.swift
   PlatformExecutorDarwin.swift
   PlatformExecutorLinux.swift
+  PlatformExecutorFreeBSD.swift
   PlatformExecutorWindows.swift
   PriorityQueue.swift
   SourceCompatibilityShims.swift

stdlib/public/Concurrency/CMakeLists.txt

@@ -164,6 +164,7 @@ set(SWIFT_RUNTIME_CONCURRENCY_SWIFT_SOURCES
   PlatformExecutorDarwin.swift
   PlatformExecutorLinux.swift
   PlatformExecutorWindows.swift
+  PlatformExecutorFreeBSD.swift
 )
 
 set(SWIFT_RUNTIME_CONCURRENCY_NONEMBEDDED_C_SOURCES

stdlib/public/Concurrency/PlatformExecutorFreeBSD.swift

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2020 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#if !$Embedded && os(FreeBSD)
+
+import Swift
+
+// The default executors for now are Dispatch-based
+@available(SwiftStdlib 6.2, *)
+public struct PlatformExecutorFactory: ExecutorFactory {
+  public static let mainExecutor: any MainExecutor = DispatchMainExecutor()
+  public static let defaultExecutor: any TaskExecutor
+    = DispatchGlobalTaskExecutor()
+}
+
+#endif // os(FreeBSD)