[compiler] Phase 3: Make lower() always produce HIRFunction (#35878)

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Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/35878).
* #35888
* #35884
* #35883
* #35882
* #35881
* #35880
* #35879
* __->__ #35878