Move ConstPtr to own GHC.ConstPtr

• Resolves #22700 (closed)
• Resolves #22699 (closed)
• Resolves #22701 (closed)

Foreign.Ptr and Foreign.C.Types are report modules, so let's not add anything to them without CLC signoff

cc @Bodigrim @bgamari @nineonine

Copy of !9646 (closed) as that MR didn't have any pipelines running.