[flang] add hlfir.product operation

Adds a HLFIR operation for the PRODUCT intrinsic according to
the design set out in flang/doc/HighLevelFIR.md

Since the PRODUCT intrinsic is essentially identical to SUM
in terms of its arguments and result characteristics in the
Fortran Standard, the operation definition and subsequent
tests also take the same form.

Differential Revision: https://reviews.llvm.org/D147624