This patch is the last of the 'firstprivate' clause lowering patches. It
takes the already generated 'copy' init from Sema and uses it to
generate the IR for the copy section of the recipe.
However, one thing that this patch had to do, was come up with a way to
hijack the decl registration in CIRGenFunction. Because these decls are
being created in a 'different' place, we need to remove the things we've
added. We could alternatively generate these 'differently', but it seems
worth a little extra effort here to avoid having to re-implement
variable initialization.
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