The ALLOCATE directive has two forms:
- A declarative form with a standalone directive:
```
!$OMP ALLOCATE (variable-list-item...)
```
- An executable form that consists of several directives followed by an
ALLOCATE statement:
```
!$OMP ALLOCATE (variable-list-item...)
!$OMP ALLOCATE (variable-list-item...)
...
ALLOCATE (...)
```
The second form was deprecated in OpenMP 5.2 in favor of the ALLOCATORS
construct.
Since in the parse tree every type corresponding to a directive only
corresponds to a single directive, the executable form is represented by
a sequence of nested OmpAllocateDirectives, e.g.
```
!$OMP ALLOCATE(x)
!$OMP ALLOCATE(y)
ALLOCATE(x, y)
```
will become
```
OmpAllocateDirective
|- ALLOCATE(x) // begin directive
`- OmpAllocateDirective // block
|- ALLOCATE(y) // begin directive
`- ALLOCATE(x, y) // block
```
With this change all AST nodes for directives use
OmpDirectiveSpecification as the directive representation.
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