[flang] OpenMP allocate directive parse tree fix

Addresses the same issue as the following abandoned revision: D104391.

Rewrite leading declarative allocations so they are nested within their respective executable allocate directive

Original:
ExecutionPartConstruct -> OpenMPDeclarativeAllocate
ExecutionPartConstruct -> OpenMPDeclarativeAllocate
ExecutionPartConstruct -> OpenMPExecutableAllocate

After rewriting:
ExecutionPartConstruct -> OpenMPExecutableAllocate
| ExecutionPartConstruct -> OpenMPDeclarativeAllocate
| ExecutionPartConstruct -> OpenMPDeclarativeAllocate

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D148409



Co-authored-by: Isaac Perry <isaac.perry@arm.com>