parse postfix attributes on asm blocks

I think this branching is not necessary, because the issue 12979 case:

```d
void foo() nothrow {
    asm { nop; }
    throw new Exception("");
}
```

Will cause:
```
test.d(3): Deprecation: asm block is not nothrow
test.d(4): Error: object.Exception is thrown but not caught
test.d(1): Deprecation: function 'test.foo' is nothrow yet may throw
```

The `throw` statement in `nothrow` function is correctly handled as error, so we can always use `error` function for the additional `is nothrow yet may throw` message.