[flang] Prepare to be merged with master branch

ActualArgument was modified in the master branch from
a struct to class. In order to compile once merged, this PR
required the related access paterns to be adapted to match
the new ActualArgument.
As a consequence, this PR does not compile anymore on its own,
it needs to be merged with the master branch to work.

Original-commit: flang-compiler/f18@48cdd2ed0ed18b9749f976257c7302e34c20b025
Tree-same-pre-rewrite: false