Clean up test comment and mark as REQUIRES: concurrency

[DougGregor]

No description provided.

[DougGregor]

@swift-ci please smoke test and merge

[davezarzycki]

Whoa. I'm surprised that GitHub didn't view our two pull requests as conflicting, so now the test looks like this:

// RUN: %empty-directory(%t)

// RUN: %target-swift-frontend -typecheck -swift-version 5 -module-name FeatureTest -emit-module-interface-path - -enable-library-evolution -enable-experimental-concurrency %s | %FileCheck %s
// REQUIRES: concurrency

// REQUIRES: concurrency

// Ensure that when we emit a Swift interface that makes use of new features,
// the uses of those features are guarded by appropriate #if's that allow older
// compilers to skip over the uses of newer features.