[flang] Allow record advancement in external formatted sequential READ

The '/' control edit descriptor causes a runtime crash for an
external formatted sequential READ because the AdvanceRecord()
member function for external units implemented only the tasks
to finish reading the current record.  Split those out into
a new FinishReadingRecord() member function, call that instead
from EndIoStatement(), and change AdvanceRecord() to both
finish reading the current record and to begin reading the next
one.

Differential revision: https://reviews.llvm.org/D88607