[flang] Runtime must defer formatted/unformatted determination

What the Fortran standard calls "preconnected" external I/O units
might not be known to be connected to unformatted or formatted files
until the first I/O data transfer statement is executed.
Support this deferred determination by representing the flag as
a tri-state Boolean and adapting its points of use.

Differential Revision: https://reviews.llvm.org/D101929