[flang] Extend & fix per-I/O-statement state (ext. I/O work part 7)

The per-I/O-statement state structures need to support missing
external I/O statements, and some bugs found in testing with
formatted input and record advancement are fixed.  The effects
of these changes will not be visible until further patches to
the I/O API handlers are pushed.

Reviewed By: tskeith

Differential Revision: https://reviews.llvm.org/D83151