Add guards for code that relies on _Concurrency (#237)

This allows Xcode 13 to still compile on macOS 11 which does not have symbols for Concurrency yet
See swift-server/swift-service-lifecycle#110

Author: programmarchy

Sources/AWSLambdaRuntimeCore/LambdaHandler.swift

@@ -88,7 +88,7 @@ extension LambdaHandler {
 
 // MARK: - AsyncLambdaHandler
 
-#if compiler(>=5.5)
+#if compiler(>=5.5) && canImport(_Concurrency)
 /// Strongly typed, processing protocol for a Lambda that takes a user defined `In` and returns a user defined `Out` async.
 @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
 public protocol AsyncLambdaHandler: EventLoopLambdaHandler {

Tests/AWSLambdaRuntimeCoreTests/LambdaHandlerTest.swift

@@ -79,7 +79,7 @@ class LambdaHandlerTest: XCTestCase {
         assertLambdaLifecycleResult(result, shoudHaveRun: maxTimes)
     }
 
-    #if compiler(>=5.5)
+    #if compiler(>=5.5) && canImport(_Concurrency)
 
     // MARK: - AsyncLambdaHandler