This patch add support for lowering of custom reductions to MLIR. It
also enhances the capability of the pass to automatically mark functions
as "declare target" by traversing custom reduction initializers and
combiners.
Add Automap modifier to the MLIR op definition for the DeclareTarget
directive's Enter clause. Also add lowering support in Flang.
Automap Ref: OpenMP 6.0 section 7.9.7.
