Only create a preprocessed file for an ignored attribute if there currently does not exist an 'attribute_ignored_XXX.txt' file for that attribute.
Only create a preprocessed file for an ignored attribute if there currently does not exist an 'attribute_ignored_XXX.txt' file for that attribute.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@65222 91177308-0d34-0410-b5e6-96231b3b80d8

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