RegionFolder should only invoke callback on free regions.

In other words, Late-bound regions that occur non-free should be
skipped.

Fix #10846.
auto merge of #15061 : pnkfelix/rust/fsk-fix-issue-10846, r=nikomatsakis

In other words, Late-bound regions that occur non-free should be
skipped.

Fix #10846 (specifically the ICE, not the weakness in the current type inference).