Example based on the gfortran test a.6.1.f90
```
  do 100 i = 1,10
  !$omp do
    do 100 j = 1,10
      call work(i,j)
    100 continue
```

During canonicalization of label-DO loops, if the body of an OpenMP
construct ends with a label, treat the label as ending the construct
itself.

This will also allow handling of cases like
```
  do 100 i = 1, 10
  !$omp atomic write
  100 x = i
```
which we were unable to before.

…ls, NFC

This patch adds semantics for the `omp fuse` directive in flang, as
specified in OpenMP 6.0. This patch also enables semantic support for
loop sequences which are needed for the fuse directive along with
semantics for the `looprange` clause. These changes are only semantic.
Relevant tests have been added , and previous behavior is retained with
no changes.

---------

Co-authored-by: Ferran Toda <ferran.todacasaban@bsc.es>
Co-authored-by: Krzysztof Parzyszek <Krzysztof.Parzyszek@amd.com>

Move `GetInnermostExecPart` and `IsStrictlyStructuredBlock` from
Semantics/openmp-utils.* to Parser/openmp-utils.*. These two only depend
on the AST contents and properties.

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.

Consider OpenMP stylized expression to be a template to be instantiated
with a series of types listed on the containing directive (currently
DECLARE_REDUCTION). Create a series of instantiations in the parser,
allowing OpenMP special variables to be declared separately for each
type.

---------

Co-authored-by: Tom Eccles <tom.eccles@arm.com>

… NFC

`GetOmpObjectList` takes a clause, and returns the pointer to the
contained OmpObjectList, or nullptr if the clause does not contain one.
Some clauses with object list were not recognized: handle all clauses,
and move the implementation to flang/Parser/openmp-utils.cpp.