Why the separate flag instead of just picking up -fobjc-nonfragile-abi and friends? Just wondering.
-fobjc-nonfragile-abi is a -cc1 option, and we'd like to be able to control the rewriter's behavior from the top-level driver in a natural way. Also, -fobjc-nonfragile-abi is controlled by the compiler's target triple, which isn't necessarily how we want to control the rewriter's behavior in all circumstances.