Add unnecessary REQUIRES: line for a feature

(cherry picked from commit 2eeb394)

Author: DougGregor

test/Concurrency/isolated_conformance_migrate.swift

@@ -1,7 +1,7 @@
 // RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.1-abi-triple -swift-version 6 -enable-upcoming-feature InferIsolatedConformances:migrate %s
 
 // REQUIRES: concurrency
-
+// REQUIRES: swift_feature_InferIsolatedConformances
 
 protocol P { }
 protocol Q: P { }