Import _Concurrency where needed (#5682)

Author: neonichu

Sources/Basics/TemporaryFile.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+import _Concurrency
 import Foundation
 import TSCBasic
 

Sources/Workspace/Workspace.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+import _Concurrency
 import Basics
 import Foundation
 import OrderedCollections