Adding some comments about AST node requirements for attributes which create AST nodes but never actually make use of them. No functional changes.
Adding some comments about AST node requirements for attributes which create AST nodes but never actually make use of them. No functional changes.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@197582 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang