[flang][OpenMP] Lowering support for atomic update construct

This patch adds lowering support for atomic update construct. A region
is associated with every `omp.atomic.update` operation wherein resides:
(1) the evaluation of the expression on the RHS of the atomic assignment
statement, and (2) a `omp.yield` operation that yields the extended value
of expression evaluated in (1).

Reviewed By: peixin

Differential Revision: https://reviews.llvm.org/D125668