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

[Matheus Izvekov via llvm-commits]

mizvekov wrote:

FYI this patch changed the default for building the rewriter to OFF.

If we want to keep this functionality around, it means this will barely get any testing, and breaking changes in API will only be caught by downstream.

Shall we switch this to default to ON?
Or maybe only change our pre-commit CI so it enables it?

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