[OpenMPIRBuilder] Support opaque pointers in reduction handling

Make the reduction handling in OpenMPIRBuilder compatible with
opaque pointers by explicitly storing the element type in ReductionInfo,
and also passing it to the atomic reduction callback, as at least
the ones in the test need the type there.

This doesn't make things fully compatible yet, there are other
uses of element types in this class. I also left one
getPointerElementType() call in mlir, because I'm not familiar
with that area.

Differential Revison: https://reviews.llvm.org/D115638