With `ARCMigrate` and `arcmt-test` removed from clang in
https://github.com/llvm/llvm-project/pull/119269 and the new code
migration experience under way (see
https://github.com/swiftlang/swift-evolution/pull/2673), these options
are no longer relevant nor known to be in use. They were introduced
long ago to support fix-it application in Xcode.
For now, turn them into a no-op and emit a obsoletion warning.
