[clang] [flang] [llvm] [Clang] Remove ARCMigrate (PR #119269)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I don't know that we've had to deal with the underling functionality for anything in libclang getting removed before; we're usually pretty conservative with what we add in the first place.

Maybe we should stub out the methods so they always fail?

https://github.com/llvm/llvm-project/pull/119269