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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I've confirmed that it's still in active use.

Thank you John! In that case, I agree that we should enable it in some CI pipeline so we get testing for it upstream.

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