[flang] add hlfir.sum operation

Add an HLFIR operation for the SUM transformational intrinsic, according
to the design set out in flang/doc/HighLevelFIR.md.

I decided to make hlfir.sum very lenient about the form of its
arguments. This allows the sum intrinsic to be lowered to only this HLFIR
operation, without needing several operations to convert and box
arguments. Having only one operation generated for the intrinsic
invocation should make optimisation passes on HLFIR simpler.

Differential Revision: https://reviews.llvm.org/D142897