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

[via llvm-commits]

Sirraide wrote:

> Return NULL, I guess; that's how the error handling for those methods used to work.

Yeah, I was just looking at what used to be there, and we’d just print that it isn’t enabled and return `NULL`; I guess we can add them back and print ‘ARCMigrate has been removed from Clang’ or sth like that.

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