[flang] Use a "double-double" accumulator in SUM

Use a "double-double" accumulator, a/k/a Kahan summation,
in the SUM intrinsic in the runtime for real & complex.
This seems to be the best-recommended technique for reducing
error, as opposed to the initial implementation of SUM's
distinct accumulators for positive and negative items.

Differential Revision: https://reviews.llvm.org/D104338