[flang][openacc] Lower copyout, detach and delete to data exit operations

Add support to lower data exit operations and use it for the
copyout, detach and delete clause on the `!$acc exit data` construct.

This patch refactor the data entry op creation so most part of the code
can be shared with the data exit op.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D149601