[flang] [llvm] [mlir] [Frontend][OpenMP] Refactor getLeafConstructs, add getCompoundConstruct (PR #87247)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

The Windows build failure is due to running out of memory:
```
fatal error C1060: compiler is out of heap space
```

https://github.com/llvm/llvm-project/pull/87247