Check alias liveness in other threads

[Mukul Rathi]

If an alias y is live in another thread, then x is not linear

let x = new Foo();
let y = x
finish{
async{
 y ....
}
x .... // alias y is live for entire finish async block
x .... 
}

[Mukul Rathi]

this doesn't actually matter (since splitting capability)