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.
Some invalid specializations were previously allowed by the compiler
and we found some existing code that used that (albeit invalid) syntax,
so we need to stage that error as a warning until Swift 6 language mode
to avoid source compatibility break.
Resolves: rdar://134740240
