[flang] Debugging of ACCESS='STREAM' I/O

Corrects the runtime implementation of I/O on files with
the access mode ACCESS='STREAM'.  This is a collection
of edge-case tweaks to ensure that the distinctions between
stream and direct/sequential files, unformatted or formatted,
are respected where appropriate.
Moves NextInField() from io-stmt.h to io-stmt.cpp --
it was getting too big to keep in a header.

Differential Revision: https://reviews.llvm.org/D118834