[flang] Fixed slice offset computation in XEmbox codegen.

For character type with unknown length we end up generating
a GEP with the base type `llvm.ptr<i[width]>`. The GEP produces
the address of the first element of the slice, and it should be
using the offset computed in the number of characters, while we were
providing the offset in bytes.

Simple reproducer fails with and w/o HLFIR:
```
program test
  integer,parameter :: ck = 4
  character(:,ck),allocatable :: res(:,:)
  allocate(character(3,ck) :: res(2,2))
  res(1,1) = ck_'111'
  res(1,2) = ck_'222'
  res(2,1) = ck_'333'
  res(2,2) = ck_'444'
  call check(res)
contains
  subroutine check(res)
    character(:,ck),allocatable :: res(:,:)
    print *, res(2,:)
  end subroutine check
end program test
```

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D156849