[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