[flang][runtime] NAMELIST input into storage sequence (#76584)

Nearly every Fortran compiler supports the extension of NAMELIST input
into a storage sequence identified by its initial scalar array element.
For example,
  &GROUP A(1) = 1. 2. 3. /
should be processed as if the input had been
  &GROUP A(1:) = 1. 2. 3. /

Fixes llvm-test-suite/Fortran/gfortran/regression/namelist_24.f90.