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.

Instead of storing a variant with specific types, store parser::Block as
the body. Add two access functions to make the traversal of the nest
simpler.

This will allow storing loop-nest sequences in the future.

…rective

This makes accessing directive components, such as directive name or the
list of clauses simpler and more uniform across different directives. It
also makes the parser simpler, since it reuses existing parsing
functionality.

The changes are scattered over a number of files, but they all share the
same nature:
- getting the begin/end directive from OpenMPLoopConstruct,
- getting the llvm::omp::Directive enum, and the source location,
- getting the clause list.

In OpenMP Version 5.1, the tile and unroll directives were added. When
using these directives, it is possible to nest them within other OpenMP
Loop Constructs. This patch enables the semantics to allow for this
behaviour on these specific directives. Any nested loops will be stored
within the initial Loop Construct until reaching the DoConstruct itself.

Relevant tests have been added, and previous behaviour has been retained
with no changes.

See also, #110008