[flang][runtime] When OPEN implies CLOSE, disable later extant unit c… (#69390)

…hecks

An OPEN statement on an existing unit can imply a CLOSE of that unit;
for example, OPEN(5, FILE="mydata", FORM="formatted") should implicitly
close the standard input that had been preconnected to unit 5. When this
happens, later checks in OPEN statement completion that apply only to
existing units should be disabled.