[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-tester/tests/unreachable-4.tolk

@@ -0,0 +1,24 @@
+fun alwaysThrows(): never {
+    throw 456;
+}
+
+fun testUnreachable(x: int) {
+    if (x) { throw 123; }
+    else { alwaysThrows(); }
+    return 1;
+}
+
+fun main() {
+    try {
+        testUnreachable(100);
+        throw 80;
+    } catch (excNo) {
+        return excNo;
+    }
+}
+
+/**
+@testcase | 0 |   | 123
+@stderr warning: unreachable code
+@stderr return 1;
+ */