[OpenACC][NFCI] Implement 'recipe' generation for firstprivate copy (#153622)
The 'firstprivate' clause requires that we do a 'copy' operation, so
this patch creates some AST nodes from which we can generate the copy
operation, including a 'temporary' and array init. For the most part
this is pretty similar to what 'private' does other than the fact that
the source is copy (and not default init!), and that there is a
temporary from which to copy.
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Co-authored-by: Andy Kaylor <akaylor@nvidia.com>