[flang] Fix actions at end of output record

It turns out that unformatted fixed-size output records
do need to be padded out if short, in order to avoid a
spurious EOF crash on a short record at the end of the file.
While here in AdvanceRecord(), move the unformatted
variable-length record header/footer writing code to here
from EndIoStatement().

Differential revision: https://reviews.llvm.org/D88685