[flang] [llvm] [Frontend][OpenMP] Follow compound construct clause restrictions (PR #107853)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

There are already checks in both flang and clang for specific cases like this.  It would be nice to have it consolidated in a single place, but the frontends handle it in their own ways (e.g. emit their own diagnostics), so doing it in a single place that accommodates the differences in the frontends may be a bit more involved.
Unless we're trying to unify handling of invalid combinations of clauses/directives, I think it's better to follow the current state.

When the OMP.td is auto-generated (in the future), it won't contain information about such restrictions.  Those are not reflected in the source (JSON) files in the spec.

https://github.com/llvm/llvm-project/pull/107853