[flang] Allow write after non advancing read in IO runtime

[flang] Allow write after non advancing read in IO runtime

1. To avoid overwriting the part of the record read in the non advancing read,
the furtherPositionInRecord field must be set to the max of the
furtherPositionInRecord and the positionInRecord at the beginning of the
IO write.

2. To allow any further read to succeed after the write, the unit
beganReadingRecord_ must be set to false when resetting the recordLength
during the write, otherwise, recordLength will not be computed in further
read and an assert is hit (at unit.cpp(398)).

The added unit test exercises both of these scenarios.

Differential Revision: https://reviews.llvm.org/D113740