fix Issue 9209 - ice(symbol.c) with const struct heap allocation
Merge pull request #1904 from WalterBright/fix9209

fix Issue 9209 - ice(symbol.c) with const struct heap allocation
Run semantic on memberless enum memtype

If enum has no members but still has a base type, eg
enum X : BaseType;
we still need to run semantic on the memtype BaseType.
Fix bug 9921: Disallow void enums. Fix enum.max for errors

If the base type isn't valid, don't calculate enum.max
Merge pull request #1907 from donc/voidenum9921

9921: Enum variables of type void should be illegal