[flang] lower product intrinsic to hlfir.product operation

Carries out the initial lowering of the product intrinsic into HLFIR
following a similar method to sum, the --use-hlfir-intrinsic-ops flag
in test/Lower/HLFIR/expr-box is set to false so that the tests will
pass until hlfir.product is lowered into fir.call

Depends on: D147624

Differential Revision: https://reviews.llvm.org/D148719