[Concurrency] @preconcurrency warnings should only come back when the imported

module has enabled Swift 6.

Warnings when the client enables Swift 6 are not useful because that's not the
point when the library API has been audited.

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -882,7 +882,9 @@ DiagnosticBehavior SendableCheckContext::diagnosticBehavior(
     if (sourceFile)
       sourceFile->setImportUsedPreconcurrency(*import);
 
-    return nominal->getASTContext().LangOpts.isSwiftVersionAtLeast(6)
+    // `@preconcurrency` suppresses diagnostics until the imported
+    // module enables Swift 6.
+    return import->module.importedModule->isConcurrencyChecked()
         ? DiagnosticBehavior::Warning
         : DiagnosticBehavior::Ignore;
   }

test/ClangImporter/predates_concurrency_import_swift6.swift

@@ -12,6 +12,6 @@
 func acceptSendable<T: Sendable>(_: T) { }
 
 func useSendable(ns: NSString) {
-  // Note: warning below is downgraded due to @preconcurrency
-  acceptSendable(ns) // expected-warning{{type 'NSString' does not conform to the 'Sendable' protocol}}
+  // Note: warning below is suppressed due to @preconcurrency
+  acceptSendable(ns)
 }

test/Concurrency/predates_concurrency_import_swift6.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -emit-module -emit-module-path %t/StrictModule.swiftmodule -module-name StrictModule -strict-concurrency=complete %S/Inputs/StrictModule.swift
+// RUN: %target-swift-frontend -emit-module -emit-module-path %t/StrictModule.swiftmodule -module-name StrictModule -swift-version 6 %S/Inputs/StrictModule.swift
 // RUN: %target-swift-frontend -emit-module -emit-module-path %t/NonStrictModule.swiftmodule -module-name NonStrictModule %S/Inputs/NonStrictModule.swift
 
 // RUN: %target-swift-frontend -swift-version 6 -I %t %s %s -emit-sil -o /dev/null -verify
@@ -15,5 +15,5 @@ func acceptSendable<T: Sendable>(_: T) { }
 @available(SwiftStdlib 5.1, *)
 func test(ss: StrictStruct, ns: NonStrictClass) {
   acceptSendable(ss) // expected-warning{{type 'StrictStruct' does not conform to the 'Sendable' protocol}}
-  acceptSendable(ns) // expected-warning{{type 'NonStrictClass' does not conform to the 'Sendable' protocol}}
+  acceptSendable(ns)
 }