[mlir][OpenMP] Change the syntax of omp.atomic.read op

This patch changes the syntax of omp.atomic.read to take the address of
destination, instead of having the value in a result. This will allow
using omp.atomic.read operation within an omp.atomic.capture operation
thus making its implementation less complex.

Reviewed By: peixin

Differential Revision: https://reviews.llvm.org/D116396