[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

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -1754,8 +1754,11 @@ struct XEmboxOpConversion : public EmboxCommonConversion<fir::cg::XEmboxOp> {
     // Adjust the element scaling factor if the element is a dependent type.
     if (fir::hasDynamicSize(seqEleTy)) {
       if (auto charTy = seqEleTy.dyn_cast<fir::CharacterType>()) {
-        prevPtrOff =
-            getCharacterByteSize(loc, rewriter, charTy, adaptor.getLenParams());
+        // The GEP pointer type decays to llvm.ptr<i[width]>.
+        // The scaling factor is the runtime value of the length.
+        assert(!adaptor.getLenParams().empty());
+        prevPtrOff = FIROpConversion::integerCast(
+            loc, rewriter, i64Ty, adaptor.getLenParams().back());
       } else if (seqEleTy.isa<fir::RecordType>()) {
         // prevPtrOff = ;
         TODO(loc, "generate call to calculate size of PDT");
@@ -1783,7 +1786,8 @@ struct XEmboxOpConversion : public EmboxCommonConversion<fir::cg::XEmboxOp> {
       // per CHARACTER element.
       auto charTy = seqEleTy.cast<fir::CharacterType>();
       if (fir::hasDynamicSize(charTy)) {
-        prevDimByteStride = prevPtrOff;
+        prevDimByteStride =
+            getCharacterByteSize(loc, rewriter, charTy, adaptor.getLenParams());
       } else {
         prevDimByteStride = genConstantIndex(
             loc, i64Ty, rewriter,