[flang] [llvm] [flang][OpenMP] Decompose compound constructs, do recursive lowering (PR #90098)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

> ```
> taskgroup.f90
> ```
This testcase is invalid: the `allocate` clause requires another clause that can privatize the given object.  The only such clause is `task_reduction`, but it's not yet supported.  The only way to fix the test now is to remove the `allocate` clause completely (https://github.com/llvm/llvm-project/pull/92173). My code also needs to recognize additional privatizing clauses (https://github.com/llvm/llvm-project/pull/92176).

https://github.com/llvm/llvm-project/pull/90098