[mlir][OpenMP] Add copyprivate support to omp.single (#80477)

This adds a new custom CopyPrivateVarList to the single operation.
Each list item is formed by a reference to the variable to be
updated, its type and the function to be used to perform the copy.

It will be translated to LLVM IR using OpenMP builder, that will
use the information in the copyprivate list to call
__kmpc_copyprivate.

This is patch 2 of 4, to add support for COPYPRIVATE in Flang.
Original PR: https://github.com/llvm/llvm-project/pull/73128