Make `=` and `const` toplevel-preserving syntax (#59784)
Previously, `map-cl-convert` would keep the toplevel flag only when the
containing expression was toplevel and each element of the list passed
to it satisfied `toplevel-preserving?`. Instead, make it so
`toplevel-preserving?` expressions are those where the toplevel flag
should be propagated to the children during closure conversion.
Fixes #59755.
