Handle '?' in vl_to_vhdl_bit

The rough translation is '-', although the semantics are incompatible
in some cases (e.g. '-' = '1' is false)

tgt-vhdl/vhdl_helper.hh

@@ -69,6 +69,8 @@ static inline char vl_to_vhdl_bit(char bit)
    case 'x':
    case 'X':
       return 'U';
+   case '?':
+      return '-';
    }
    assert(false);
 }