[flang] Fix bug in rewriting function calls

Arrays are now represented with ObjectEntityDetails, not EntityDetails.
So that's what we have to look for when rewriting function calls to
array element references.

Original-commit: flang-compiler/f18@841b37d94bc02f5a8726873fff7182cee053c8c3
Reviewed-on: https://github.com/flang-compiler/f18/pull/177
Tree-same-pre-rewrite: false