[APIDigester] Lift the requirement that -sdk must be present

Author: owenv

tools/driver/swift_api_digester_main.cpp

@@ -2365,10 +2365,6 @@ static void setSDKPath(CompilerInvocation &InitInvok, bool IsBaseline,
  InitInvok.setSDKPath(SDK.str());
  } else if (const char *SDKROOT = getenv("SDKROOT")) {
  InitInvok.setSDKPath(SDKROOT);
- } else {
- llvm::errs() << "Provide '-sdk <path>' option or run with 'xcrun -sdk <..>\
- swift-api-digester'\n";
- exit(1);
  }
  }
 }