`fn destroy` is being removed per decision on #6124. It seems like the
relevant decision will result in no more keyword-based function names,
so this is removing all related support.
Attach the cleanup to the `Temporary` instruction instead of to the
`TemporaryStorage` instruction. We create `TemporaryStorage`
instructions speculatively when creating an initializing expression, and
may overwrite those instructions with other instructions if it turns out
that a temporary is not required. Instead, wait until we finalize the
temporary and create a `Temporary` instruction to register the cleanup.
Jon Ross-Perkins
Richard Smith