[flang][OpenMP] Use OmpDirectiveSpecification in ALLOCATE (#165865)

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.

0 files changed, 0 insertions, 0 deletions