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

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

In OMP.td the clauses on compound constructs should be unions of clauses from the leaf constructs. This is the default assumption in the spec. Special case restrictions, like the ones from 17.4 should be handled in separate checks.

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