Added lowering support for atomic read and write constructs

This patch adds lowering support for atomic read and write constructs.
Also added is pointer modelling code to allow FIR pointer like types to
be inferred and converted while lowering.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D122725



Co-authored-by: Kiran Chandramohan <kiran.chandramohan@arm.com>