[flang][OpenMP] Threadprivate variable in host-association

[PeixinQiao]

flang-compiler#1136 has the detailed info about the support status of Threadprivate directive.

Test case

$ cat hostassoc1.f90 
program main
  use omp_lib
  integer, save :: a
  !$omp threadprivate(a)
  call sub()
contains
  subroutine sub()
    a = 2
    !$omp parallel num_threads(4)
      a = omp_get_thread_num()
      print *, a, omp_get_thread_num()
    !$omp end parallel
    !$omp parallel num_threads(4)
      print *, a, omp_get_thread_num()
    !$omp end parallel
  end
end
$ cat hostassoc2.f90 
program main
  use omp_lib
  integer :: a
  !$omp threadprivate(a)
  call sub()
contains
  subroutine sub()
    a = 2
    !$omp parallel num_threads(4)
      a = omp_get_thread_num()
      print *, a, omp_get_thread_num()
    !$omp end parallel
    !$omp parallel num_threads(4)
      print *, a, omp_get_thread_num()
    !$omp end parallel
  end
end

Expected results:

           0           0
           2           2
           3           3
           1           1
           2           2
           3           3
           1           1
           0           0

GFortran 12 and ifort 2021 support hostassoc1.f90, but do not support hostassoc2.f90. For hostassoc2.f90, GFortran 12 gives wrong running results, while ifort reports the semantic error.

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