Wrap enclosing loop body when a nested for-head closure captures a let binding - #18337

transform-block-scoping lowers a nested `for (let x = ...)` to `var`, so
a closure created inside that initializer captured one function-scoped
binding across every iteration of the enclosing loop. The loop-body
binding collector skipped nested loops entirely, so the enclosing loop
never learned that its body needed a per-iteration wrapper.

Collect a nested ForStatement's let/const head bindings when a closure in
its initializer captures them, so the enclosing loop wraps its body and
each entry of the nested loop gets a fresh binding. The existing
for-head-closure-nested fixture now asserts the spec result instead of
the TODO.

Fixes babel#18200

Assisted-by: Claude Fable 5.1