[Flang][OpenMP] Add support for integer multiplication reduction in worksharing-loop

Adds support for reduction of multiplcation
by extending OpenMP.cpp::genOpenMPReduction()
and altering the identity constant emitted in
OpenMP.cpp::createReductionDelc()

This patch builds D130077 and as such,
only supports reductions for interger types in
worksharping loops.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D130767