[flang] Fix output buffering bug (positionability assumption)

The I/O runtime library code was failing to retain data in a buffer
from the current output record when flushing the buffer; this is
fatally wrong when the corresponding file cannot be repositioned,
as in the case of standard output to the console. So refine the
Flush() member function to retain a specified number of bytes,
rearrange the data as necessary (using existing code for read frame
management after moving it into a new member function), and add
a big comment to the head of the file to clarify the roles of the
various data members in the management of contiguous frames in
circular buffers.

Update: added a unit test.

Differential Revision: https://reviews.llvm.org/D99198