Fix Apple simulator and Swift 6 build plumbing

Apple/Configuration/Compiler.xcconfig

@@ -40,5 +40,4 @@ APP_GROUP_IDENTIFIER = group.$(APP_BUNDLE_IDENTIFIER)
 APP_GROUP_IDENTIFIER[sdk=macosx*] = $(DEVELOPMENT_TEAM).$(APP_BUNDLE_IDENTIFIER)
 NETWORK_EXTENSION_BUNDLE_IDENTIFIER = $(APP_BUNDLE_IDENTIFIER).network
 
-// https://github.com/grpc/grpc-swift/issues/683#issuecomment-1130118953
-OTHER_SWIFT_FLAGS = $(inherited) -Xcc -fmodule-map-file=$(GENERATED_MODULEMAP_DIR)/CNIOAtomics.modulemap -Xcc -fmodule-map-file=$(GENERATED_MODULEMAP_DIR)/CNIODarwin.modulemap -Xcc -fmodule-map-file=$(GENERATED_MODULEMAP_DIR)/CGRPCZlib.modulemap
+OTHER_SWIFT_FLAGS = $(inherited)

Apple/Configuration/Constants/Constants.swift

@@ -1,4 +1,5 @@
 @_implementationOnly import CConstants
+import Foundation
 import OSLog
 
 public enum Constants {
@@ -27,9 +28,30 @@ public enum Constants {
     private static let _groupContainerURL: Result<URL, Error> = {
         switch FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: appGroupIdentifier) {
         case .some(let url): .success(url)
-        case .none: .failure(.invalidAppGroupIdentifier)
+        case .none:
+            fallbackContainerURL().mapError { _ in .invalidAppGroupIdentifier }
         }
     }()
+
+    private static func fallbackContainerURL() -> Result<URL, any Swift.Error> {
+#if targetEnvironment(simulator)
+        Result {
+            let baseURL = try FileManager.default.url(
+                for: .applicationSupportDirectory,
+                in: .userDomainMask,
+                appropriateFor: nil,
+                create: true
+            )
+            let url = baseURL
+                .appending(component: bundleIdentifier, directoryHint: .isDirectory)
+                .appending(component: "SimulatorFallback", directoryHint: .isDirectory)
+            try FileManager.default.createDirectory(at: url, withIntermediateDirectories: true)
+            return url
+        }
+#else
+        .failure(Error.invalidAppGroupIdentifier)
+#endif
+    }
 }
 
 extension Logger {