[OMPIRBuilder] add minimalist reduction support

This introduces a builder function for emitting IR performing reductions in
OpenMP. Reduction variable privatization and initialization to the
reduction-neutral value is expected to be handled separately. The caller
provides the reduction functions. Further commits can provide implementation of
reduction functions for the reduction operators defined in the OpenMP
specification.

This implementation was tested on an MLIR fork targeting OpenMP from C and
produced correct executable code.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D104928