[mlir][llvm] Fix access group translation (#83257)

This commit fixes the translation of access group metadata to LLVM IR.
Previously, it did not use a temporary metadata node to model the
placeholder of the self-referencing access group nodes. This is
dangerous since, the translation may produce a metadata list with a null
entry that is later on changed changed with a self reference. At the
same time, for example the debug info translation may create the same
uniqued node, which after setting the self-reference the suddenly
references the access group metadata. The commit avoids such breakages.