Case fallthough for multivalued cases is an error now

This used to give a deprecation and now gives an error:
```
int i;
switch (0)
{
    case 0, 1: i = 20;
    default: assert(0); // Error: switch case fallthrough - use 'goto default;' if intended
}

switch (0)
{
    default:
    case 0, 1: i = 20;
    case 2, 3: i = 30; // Error: switch case fallthrough - use 'goto case;' if intended
}
```
