[Flang][OpenMP] NFC: Refactor privatization code in Lowering 1/n

The code for handing privatization code has grown in complexity.
This patch begins an effort to refactor and simplify the code.
The refactoring will hopefully will make it modular and
enable privatization to be invoked in different places. This
patch moves the privatization related code into a class called
dataSharingProcessor.

Reviewed By: DavidTruby

Differential Revision: https://reviews.llvm.org/D144766