If there are only declarations, use the new function as the initializer
for a destructuring declaration.
If there are declarations and writes, changes all of the `const`
declarations to `let` and add `| undefined` onto any explicit types.
Use destructuring assignment to accomplish both "initialization" and
writes.
I don't believe there is a case where there are both declarations and a
return (since the declarations wouldn't be available after the return).
UNDONE: this could probably be generalized to handle binding patterns
but,
for now, only identifiers are supported.
Fixes#18242Fixes#18855
568c8a3298