[flang][OpenMP] Null Symbol reference in ResolveSeqLoopIndexInParallelOrTaskConstruct for an undeclared I/O implied-DO index under IMPLICIT NONE inside PARALLEL DEFAULT(NONE) #239946

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1. YuanchengJiang

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   Author: Jiang Yuancheng (YuanchengJiang)

   Details

   The following code: ```fortran subroutine s implicit none real :: a(10) !$omp parallel default(none) shared(a) read(5,*) (a(i),i=1,10) !$omp end parallel end ```

   Resulted in this output:

   flang-24: error: unable to execute command: Segmentation fault (core dumped)
   Stack dump:
    #<!-- -->3 Fortran::semantics::OmpAttributeVisitor::ResolveSeqLoopIndexInParallelOrTaskConstruct
    #<!-- -->4 void Fortran::common::log2visit::Log2VisitHelper
    #<!-- -->5 void Fortran::common::log2visit::Log2VisitHelper
    #<!-- -->6 void Fortran::common::log2visit::Log2VisitHelper
    #<!-- -->7 void Fortran::parser::detail::ParseTreeVisitorLookupScope::Walk
    #<!-- -->8 void Fortran::parser::detail::ParseTreeVisitorLookupScope::ForEachInTuple
    #<!-- -->9 void Fortran::common::log2visit::Log2VisitHelper
   #<!-- -->10 void Fortran::parser::detail::ParseTreeVisitorLookupScope::Walk
   

   Trunk reproducer (-fsyntax-only -fopenmp): https://godbolt.org/z/G136daxcb