[flang] Track known file size, add IsATerminal (ext. I/O work part 5)

Add a data member knownSize_ and an accessor to allow the size of
an external file to be tracked when known.  Also add a wrapper for
::isatty() here in the filesystem encapsulation module.  These
features are needed for the external I/O rework changes still
to come.

Reviewed By: sscalpone

Differential Revision: https://reviews.llvm.org/D83141