[flang][runtime] Resilient opening of anonymous unit (#93876)

When an I/O statement references a unit number that has not been
explicitly opened or predefined, the I/O runtime support library opens a
local "fort.N" file. If this fails, the program crashes, even when the
I/O statement has IOSTAT= or IOMSG= or ERR= control list items. Connect
the dots to enable resilience in these cases.