dillon1000/react

[rust][sema] Statically detect some TDZ violations

The [temporal dead 
zone](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#temporal_dead_zone_tdz), 
often abbreviated TDZ, is the period between the start of its declaring block 
and the line that contains the let/const/class declaration. Not all instances of 
TDZ can be detected statically, because whether or not a TDZ error will occur at 
runtime is a property of which control flow path is taken and whether some other 
code has initialized the value. However, a subset of cases can be detected, and 
that's what we implement here. 

When we encounter a variable reference we record the next declaration id at that 
point in time. Then when resolving references after visiting the program, we can 
check: did the reference end up referring to a let/const/class binding whose id 
is equal or greater to that "next declaration"? If so, it means the reference 
refers to a variable that is provably declared later and is a known TDZ 
violation. The catch is that when resolving references, we reset the "next 
declaration" limit value when we bubble up out of a function scope. That's 
because references to let/const within a function may occur after the 
declaration, and we can't statically validate them.