Raise error if impossible default case is reached.

[MortimerSnerd]

Summary

In the code generated by a compiler for an exhaustive case statement,
it would be helpful for debugging if a default case was emitted that just raises
something like newException(Defect, "Impossible case value").

Description

As an example:

   type
      Kinds = enum
         kThis, kThat
   
   proc doSomething(k: Kinds) = 
      case k
      of kThis:
          echo "this"
      of kThat:
        echo "that"

For the C++ backend, it generates a switch statement with a case
with two branches, one for each enum value. Normally, this is sufficient,
because k can't take on an invalid value - attempts to cast an out of range int
to Kinds will just raise an exception.

This can be a problem if you're working with pointers though. For example,
if the function was:

proc doSomething(k: ptr Kinds) = 
   case k[]
   of ...

Since the memory pointed to by k[] can be initialized outside of the safety constraints of the language, k[] can be out of range of the enum, and no branch of the switch
will be taken. What I'd like for this case is for the switch to have a default branch that
just raises a Defect exception saying something like "Invalid value for case".

Of course this came up because I had a pointer to an incorrectly initialized block of
memory. It took me quite a few rounds of echo debugging to figure out that none of
the branches of my case statement were being executed, and why.

[cooldome]

This would be a noticeable overhead in the release mode, but could be useful in debug mode.
Likely it is not the case that needs extra default clause, but enum value checks instead.