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

[via llvm-commits]

Sirraide wrote:

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

That was intentional because the vast majority of people don’t need it, so building it would just mean wasting time compiling code that never runs.

> 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?

If that’s possible then I’d prefer only enabling it in CI; as I understand it the rewriter is sort of an obsolescent feature that will be removed eventually anyway, but I might be wrong about that.

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