[flang] Fix READ/WRITE with POS= on stream units, with refactoring

First, ExternalFileUnit::SetPosition was being used both as a utility
within the class' member functions as well as an API from I/O statement
processing.  Make it private, and add APIs for SetStreamPos and SetDirectRec.

Second, ensure that SetStreamPos for POS= positioning in a stream
doesn't leave the current record number and endfile record number
in an arbitrary state.  In stream I/O they are used only to manage
end-of-file detection, and shouldn't produce false positive results
from IsAtEnd() after repositioning.

Differential Revision: https://reviews.llvm.org/D128388