[clang] [clang][OpenMP] Diagnose badly-formed collapsed imperfect loop nests (#60678) (PR #101305)

[Alexey Bataev via cfe-commits]

alexey-bataev wrote:

> OpenMPIterationSpaceChecker is still passed a pointer to CollapsedLoopDecls, because one caller passes a nullptr, and we don't want to do the analysis in that case.

Still pass by reference, just pass empty where it is not required

https://github.com/llvm/llvm-project/pull/101305