Only declare isolation macro when the compiler supports macros

Author: DougGregor

stdlib/public/Concurrency/Actor.swift

@@ -73,8 +73,11 @@ public func _defaultActorDestroy(_ actor: AnyObject)
 @usableFromInline
 internal func _enqueueOnMain(_ job: UnownedJob)
 
+#if $Macros
 /// Produce a reference to the actor to which the enclosing code is
 /// isolated, or `nil` if the code is nonisolated.
 @available(SwiftStdlib 5.1, *)
 @freestanding(expression)
 public macro isolation() -> (any Actor)? = Builtin.IsolationMacro
+#endif
+