Emit -enable-experimental-concurrency into module interfaces

…when the module is built with that flag. Fixes rdar://69322538.

test/ModuleInterface/concurrency.swift

@@ -0,0 +1,21 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -typecheck -enable-library-evolution -enable-experimental-concurrency -emit-module-interface-path %t/Library.swiftinterface -DLIBRARY -module-name Library %s
+
+#if LIBRARY
+public func fn() async {
+  fatalError()
+}
+
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency -I %t
+
+#else
+import Library
+
+func callFn() async {
+  await fn()
+}
+#endif
+
+// RUN: %FileCheck %s <%t/Library.swiftinterface
+// CHECK: // swift-module-flags:{{.*}} -enable-experimental-concurrency
+// CHECK: public func fn() async