[flang] Fix semantic checks for C919

The previous semantic analysis does not consider when the last part-ref
is scalar or complex part. Refactor the previous code and bring all the
checks into one place. The check starts from the designator by
extracting the dataref wrapped including the substring and complex part
and recursively check the base objects.

Co-authored-by: Peter Klausler <pklausler@nvidia.com>

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D126595