[flang][runtime] Crash more informatively in defined I/O error case (#74134)

Defined unformatted I/O is not allowed except from/to an external unit.
This restriction prohibits an INQUIRE(IOLENGTH=n) statement from using
derived types with defined unformatted output in its I/O list. The
runtime currently detects this case and crashes with an internal error;
this patch defines a new I/O error enum and causes the program to crash
with a more useful message.