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

[via llvm-commits]

Sirraide wrote:

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

As in make them do nothing essentially (i.e. return `NULL` etc.) or as in report a fatal error and exit?

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