[llvm] Reapply "[Dexter] Add rewriting for aggregate variables (#202800)" (PR #206535)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

Merging with post-commit review, since the code content of this patch is identical to the original patch and the pre-merge CI checks, which previously did not run on this PR but failed when they did run for other PRs, are now both running and passing on this PR: https://github.com/llvm/llvm-project/actions/runs/28456679095/job/84333275317?pr=206535

https://github.com/llvm/llvm-project/pull/206535