The ABISysV ABI was refactored in r364216 to support the Windows ABI for x86_64. In particular it changed ABISysV_x86_64::CreateInstance to switch on the OS type. This breaks debugging MacABI apps as well as apps in the simulator. This adds back the necessary cases.
We have a test on Github that exercises this code path and which I'd like to upstream once the remaining MacABI parts become available in clang.