[clang] [Sema] Return RecoveryExpr for ARC atomic ownership error (PR #224849)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

We usually don't do codegen for code that triggers an "unrecoverable" error, as denoted by DiagnosticIDs::isUnrecoverable().  Most ObjC-ARC errors are not considered "unrecoverable" for some historical reason I don't know, maybe something related to the ARCMigrate tool.

We should either make err_arc_atomic_ownership unrecoverable, or we should generate an AST that CodeGen can understand.

At this point, the tool is gone, so we can probably just remove the special rule for ARC errors.

https://github.com/llvm/llvm-project/pull/224849