[flang][OpenMP] Semantic checks for metadirective loop nests (#207088)

A loop-associated metadirective variant (`do`, `simd`, ...) is only
resolved during lowering, so it is never checked as a loop construct
during semantic analysis. A malformed or non-canonical associated nest
therefore reaches lowering, which assumes a canonical nest.

This patch validates the nest that follows such a variant (the next
executable construct) during semantics, reusing the diagnostics of a
real loop-associated construct. Each applicable variant is checked
against it:

* Canonical loop: the affected loop must be a canonical DO loop, so a
`DO WHILE`, a pre-6.0 `DO CONCURRENT`, or a `DO` without loop control is
rejected.
* Nest depth: `collapse(n)` and `ordered(n)` must not exceed the depth
of the associated loop nest.
* Rectangularity: loops that must be rectangular (e.g. under `tile`) may
not have bounds that depend on an outer loop's variable.

A loop-associated variant may also have no loop nest at all. This
happens when the metadirective ends the execution part or a non-loop
construct follows it. Such a variant is rejected like a bare loop
directive with no associated loop.

Only variants that could be selected are checked: when a target is
known, a WHEN variant whose device or implementation selector cannot
match it is skipped (runtime `user` conditions never cause a skip).

Part of the feature work for #188820. Builds on top of
[#193664](https://github.com/llvm/llvm-project/issues/193664) and
[#194424](https://github.com/llvm/llvm-project/pull/194424).

Assisted with Copilot.