[flang][runtime] Keep frame buffer in sync with file when truncating

When the I/O runtime is truncating an external file due to an
implied ENDFILE or explicit ENDFILE, ensure that the unit's frame
buffer for the file discards any data that have become obsolete.

This bug caused trouble with ACCESS='STREAM' I/O using POS= on
a WRITE, but it may have not been limited to that scenario.

Differential Revision: https://reviews.llvm.org/D129673