[flang][OpenMP] Fix construct privatization in default clause (#72510)

Current implementation of default clause privatization incorrectly fails
to privatize in presence of non-OpenMP constructs (i.e. nested
constructs with regions whose symbols need to be privatized in the scope
of the parent OpenMP construct). This patch fixes the same by
considering non-OpenMP constructs separately by collecting symbols of a
nested region if it is a non-OpenMP construct with a region, and
privatizing it in the scope of the parent OpenMP construct.

Fixes https://github.com/llvm/llvm-project/issues/71914 and
https://github.com/llvm/llvm-project/issues/71915