[flang] Set right "inNamelist" flag

NAMELIST I/O was inconsistent in its choice of which set of I/O modes
to set the "inNamelist" flag.  The wrong choice was in the set of modes
that are part of the persistent state of an I/O connection; the right
place is the set of modes that are reinitialized at the beginning of
each I/O statement so that they can be modified by READ/WRITE control
list specifiers and FORMAT control edit descriptors.  Fix.

Differential Revision: https://reviews.llvm.org/D118745