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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> `test/Rewrite` is for the Objective-C => MSVC-compatible C++ rewriter (`-rewrite-objc`), which is a different feature. That feature might _also_ be obsolete, but we haven't signed off on that yet AFAIK.

If cannot drop CLANG_ENABLE_OBJC_REWRITER code yet, defaulting to off but enabling it on pre-commit CI looks good to me.
```
option(CLANG_ENABLE_OBJC_REWRITER "Build the Objective-C rewriter tool" OFF)
```

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