[Tolk] throw interrupts control flow; never type

In FunC (and in Tolk before) throwing an exception is just
calling a built-in function:
> throw 123; // actually, __throw(123)
Since it's a regular function, the compiler was not aware
that execution will stop, and all following code is unreachable.
For instance, `throw` in the end on function needed to be
followed by `return` statement.

Now, `throw` interrupts control flow, all statements after
it are considered unreachable. At IR level, code Ops are
also not produced.

This works because a built-in __throw() now has `never` type.
It can also be applied to custom functions:
> fun alwaysThrow(): never { throw 123; }
The code after alwaysThrow() call will also be unreachable.

tolk/smart-casts-cfg.h

@@ -77,6 +77,7 @@ struct SinkExpression {
 enum class UnreachableKind {
   Unknown,     // no definite info or not unreachable
   CantHappen,
+  ThrowStatement,
   ReturnStatement,
   CallNeverReturnFunction,
 };