[flang] Complete the fix for NAMELIST look-ahead case

When processing the devious NAMELIST input

  &group logarray = t t t
    = 666 /

for LOGICAL::logarray(3) and INTEGER::t, the runtime library
needs to do some look-ahead on the input stream to make sure
that the last "t" on the first line is a truth value rather than
an item name -- which in this case it is.  This look-ahead
was implemented in a previous patch but only worked for internal
input cases; this patch implements look-ahead capabilities for
input from an external file, too (and also adjusts repeated
list-directed input items to use this infrastructure, too).

Differential Revision: https://reviews.llvm.org/D113311