[Coroutines] Fix a warning

This patch fixes:

  llvm/lib/Transforms/Coroutines/Coroutines.cpp:475:3: error: default
  label in switch which covers all enumeration values
  [-Werror,-Wcovered-switch-default]

Author: kazutakahirata

llvm/lib/Transforms/Coroutines/Coroutines.cpp

@@ -472,8 +472,6 @@ void coro::Shape::initABI() {
     }
     break;
   }
-  default:
-    llvm_unreachable("coro.begin is not dependent on a coro.id call");
   }
 }