[flang] Fix WRITE after BACKSPACE

Authored by klausler on Oct 1 2020, 9:50 AM.


[flang] Fix WRITE after BACKSPACE

A WRITE to an unformatted sequential variable-length unit after
a BACKSPACE needs to forget its previous knowledge of the length
of the record that's about to be overwritten, and a BACKSPACE
after an ENDFILE or at the start of the file needs to be a no-op.

Differential revision: https://reviews.llvm.org/D88675