[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