Add comments. NFC
Add comments. NFC

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@279490 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit e8abf261c4b533e3f07cf2fc64590dcdd4d8cb6f)

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