[mlir][OpenMP] Lower REDUCTION clause for SECTIONS construct (llvm#97859

)

This shares code with WsloopOp (the changes to Wsloop should be NFC).
OpenMPIRBuilder basically implements SECTIONS as a wsloop over a case
statement with each SECTION as a case for a particular loopiv value.

Unfortunately it proved very difficult to share code between these and
ParallelOp. ParallelOp does quite a few things differently (doing more
work inside of the bodygen callback and laying out blocks differently).
Aligning reduction implementations for wsloop and parallel will probably
involve functional changes to both, so I won't attempt that in this
commit.