[flang][runtime] Correct emission & reading of unterminated final records

When the last operation on a foramtted sequential or stream file (prior
to an implied or explicit ENDFILE) is a non-advancing WRITE, ensure
that any partial record data is emitted to the file without a line
terminator.  Further, when that last record is read with a non-advancing
READ, ensure that it won't raise an end-of-record condition after its
data, but instead will signal an end-of-file.

Differential Revision: https://reviews.llvm.org/D124546