[flang][OpenMP] Adapt OMPMapInfoFinalization to run on all top level ops (#93545)

This is generally just for consistency with the rest of the pipeline.

The assertion for the insertion point is because I am not sure if
omp::PrivateClauseOp is supported by FirOpBuilder::getAllocaBlock. I
didn't try to fix it because I don't see why we would generate IR like
that.

See RFC:
https://discourse.llvm.org/t/rfc-add-an-interface-for-top-level-container-operations