Previously, loop constructs were parsed in a piece-wise manner: the
begin directive, the body, and the end directive were parsed separately.
Later on in canonicalization they were all coalesced into a loop
construct. To facilitate that end-loop directives were given a special
treatment, namely they were parsed as OpenMP constructs. As a result
syntax errors caused by misplaced end-loop directives were handled
differently from those cause by misplaced non-loop end directives.

The new loop nest parser constructs the complete loop construct,
removing the need for the canonicalization step. Additionally, it is the
basis for parsing loop-sequence-associated constructs in the future.

It also removes the need for the special treatment of end-loop
directives. While this patch temporarily degrades the error messaging
for misplaced end-loop directives, it enables uniform handling of any
misplaced end-directives in the future.

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>

Add support for the standalone OpenMP tile construct:
```f90
!$omp tile sizes(...)
DO i = 1, 100
  ...
```

This is complementary to #143715 which added support for the tile
construct as part of another loop-associated construct such as
worksharing-loop, distribute, etc.