asEP_DISALLOW_VALUE_ASSIGN_FOR_REF_TYPE support

[Paril]

As a workspace option, I'd love to be able to turn this one on; I don't actually use this option because I need it for certain behaviors, but I would love if the langserv could report errors as if it was enabled so I can catch any land-mines.

class Test
{
int y;
Test() { }
}

Test x, z;
Test @y = x;
y = z; // allowed, but I'd like this to generate an error