[API Notes] Suppress an unused-variable warning when assertions are disabled.
[API Notes] Suppress an unused-variable warning when assertions are disabled.

Fixes rdar://problem/28589320.

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang