[flang][hlfir] lower hlfir.sum into fir runtime call

This duplicates some argument lowering and return value processing from
flang/lib/Lower/ConvertCall.cpp. The existing code in CovertCall lowers
directly into the fir.call (without the hlfir transformational intrinsic
operation), and it is too tied to the lowering code to move into
flang/lib/Optimizer to allow for more complete re-use here.

Differential Revision: https://reviews.llvm.org/D143512