Generate warning for enum select statements that miss values

[bradcray]

This issue requests that select statements over enums that are incomplete should generate a warning or error:

enum clrs { RED, GREEN, BLUE };
var c = clrs.GREEN;

select c {
  when clrs.RED do writeln("c is RED");
  when clrs.BLUE do writeln("c is BLUE");
  // compilation warning expected: GREEN not handled
}

[converting to an issue as part of #9222]

In test/users/kreider/feature_select.chpl (#2060), @PriMachVisSys requested a compiler warning for select statements over enums that do not have cases for each of the enum's values. I'm capturing this as an issue here as part of my effort w.r.t. #9222.

[ronawho]

Thematically related to https://github.com/chapel-lang/chapel/issues/10192 (don't error about missing returns for exhaustive select stmts)

For both issues I imagine the underlying analysis will be the same.

[bradcray]

Thanks, I meant to link them, but then forgot by the time I'd written it up (I was disappointed to find that the .future and your issue differed since it meant I had to write this up :) ).