[flang][runtime] Fix BACKSPACE-WRITE on variable-length unformatted file (#72732)

A subtle bug in buffer management is being caused by a WRITE on an
unformatted file with variable-length records after one or more
BACKSPACEs and some READs. An attempt at a minor optimization in
BACKSPACE kept the footer of the previous record in the unit's buffer,
in case more BACKSPACEs were to follow. If a later WRITE takes place
instead, the buffer's frame would still cover that footer, but its
content would be lost when the buffer became dirty on its first
modification, and the footer in the file would be overwritten with stale
buffer contents. As WriteFrame() implies the intent to define all the
bytes in the identified range, the trick being used in BACKSPACE with
its adjustment to frameOffsetInFile_ breaks any later WRITE... and so we
just can't do it.

Fixes https://github.com/llvm/llvm-project/issues/72599.

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