[flang][OpenMP] Added allocate clause translation for OpenMP block constructs

This patch adds translation for allocate clause for parallel and single
constructs.

Also added tests for block constructs.

This patch also adds tests for parallel construct which were not added earlier.

Reviewed By: NimishMishra, peixin

Differential Revision: https://reviews.llvm.org/D122483



Co-authored-by: Sourabh Singh Tomar <SourabhSingh.Tomar@amd.com>