[Flang] Add a factory class for creating Complex Ops

Use the factory class in the FIRBuilder.
Add unit tests for the factory class function and the convert function
of the Complex class.

Reviewed By: clementval, rovka

Differential Revision: https://reviews.llvm.org/D114125



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>