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
The single source file reduction.cpp is a little large in
terms of both source lines and generated text bytes, so
split SUM, PRODUCT, FINDLOC, and MAXLOC/MAXVAL/MINLOC/MINVAL
off into their own C++ source files that share a set of
implementation function templates now in a common header.
Differential Revision: https://reviews.llvm.org/D101111
