[LTO] Move callback member from base class to the derived where it is used (NFC)
[LTO] Move callback member from base class to the derived where it is used (NFC)

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@279212 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm