Add the reverse directive which will be introduced in the upcoming
OpenMP 6.0 specification. A preview has been published in [Technical
Report 12](https://www.openmp.org/wp-content/uploads/openmp-TR12.pdf).
---------
Co-authored-by: Alexey Bataev <a.bataev@outlook.com>
OpenMP loop transformation did not work on a for-loop using an iterator
or range-based for-loops. The first reason is that it combined the
iterator's type for generated loops with the type of `NumIterations` as
generated for any `OMPLoopBasedDirective` which is an integer. Fixed by
basing all generated loop variables on `NumIterations`.
Second, C++11 range-based for-loops include syntactic sugar that needs
to be executed before the loop. This additional code is now added to the
construct's Pre-Init lists.
Third, C++20 added an initializer statement to range-based for-loops
which is also added to the pre-init statement. PreInits used to be a
`DeclStmt` which made it difficult to add arbitrary statements from
`CXXRangeForStmt`'s syntactic sugar, especially the for-loops init
statement which does not need to be a declaration. Change it to be a
general `Stmt` that can be a `CompoundStmt` to hold arbitrary Stmts,
including DeclStmts. This also avoids the `PointerUnion` workaround used
by `checkTransformableLoopNest`.
End-to-end tests are added to verify the expected number and order of
loop execution and evaluations of expressions (such as iterator
dereference). The order and number of evaluations of expressions in
canonical loops is explicitly undefined by OpenMP but checked here for
clarification and for changes to be noticed.
Allow non-constants in the `sizes` clause such as
```
#pragma omp tile sizes(a)
for (int i = 0; i < n; ++i)
```
This is permitted since tile was introduced in [OpenMP
5.1](https://www.openmp.org/spec-html/5.1/openmpsu53.html#x78-860002.11.9).
It is possible to sneak-in negative numbers at runtime as in
```
int a = -1;
#pragma omp tile sizes(a)
```
Even though it is not well-formed, it should still result in every loop
iteration to be executed exactly once, an invariant of the tile
construct that we should ensure. `ParseOpenMPExprListClause` is
extracted-out to be reused by the `permutation` clause of the
`interchange` construct. Some care was put into ensuring correct behavior
in template contexts.
