[Flang][OpenMP] Fix HLFIR lowering for commonblock threadprivate

Commonblock names are not variables, but they can be marked as
threadprivate in OpenMP. This requires the commonblock name to
be bound to the address of the Commonblock. hlfir.declares are
not required for these, but we should be able to retrieve the
mlir Value corresponding to the Commonblock. This patch enables
this by special casing the Commonblocks like procedures.

Reviewed By: tblah, vzakhari

Differential Revision: https://reviews.llvm.org/D158070