Add Sendable conformance to FileDescriptor

Sources/System/FileDescriptor.swift

@@ -476,10 +476,11 @@ extension FileDescriptor.OpenOptions
 }
 
 #if compiler(>=5.5) && canImport(_Concurrency)
-// The decision on whether to make FileDescriptor Sendable or not
-// is currently being discussed in https://github.com/apple/swift-system/pull/112
-//@available(*, unavailable, message: "File descriptors are not completely thread-safe.")
-//extension FileDescriptor: Sendable {}
+// File descriptors aren't necessarily safe to use across threads.
+// However, since they can be used in a safe way,
+// we do make them `Sendable` to not make it unnecessarily complicated to
+// use them across concurrency boundaries in a safe way.
+extension FileDescriptor: Sendable {}
 
 @available(/*System 0.0.1: macOS 11.0, iOS 14.0, watchOS 7.0, tvOS 14.0*/iOS 8, *)
 extension FileDescriptor.AccessMode: Sendable {}