[flang][hlfir] probe ultimate intrinsic module procedure name

HLFIR lowering was currently hitting errors like "not yet implemented:
intrinsic: z0$iso_c_binding$c_associated_c_ptr". These error were caused
by HLFIR lowering using the name with the full context instead of
the ultimate one.
I do not want to change proc.GetName() because I think it may be best
if it retains the full context for its usages in semantics, so I fixed
the helper to get the name in lowering to HLFIR.

Differential Revision: https://reviews.llvm.org/D154230