Sebastian Graf authored Jan 07, 2021 and Marge Bot committed Jan 10, 2021

Since !4493 we annotate top-level bindings with demands, which leads to
novel opportunities for thunk splitting absent top-level thunks.

It turns out that thunk splitting wasn't quite equipped for that,
because it re-used top-level, `External` Names for local helper Ids.
That triggered a CoreLint error (#19180), reproducible with `T19180`.

Fixed by adjusting the thunk splitting code to produce `SysLocal` names
for the local bindings.

Fixes #19180.

Metric Decrease:
    T12227
    T18282