Auto merge of #139287 - compiler-errors:folder-experiment-1, r=lqd

Folder experiment: Monomorphize region resolver

**NOTE:** This is one of a series of perf experiments that I've come up with while sick in bed. I'm assigning them to lqd b/c you're a good reviewer and you'll hopefully be awake when these experiments finish, lol.

r? lqd

This is actually two tweaks to the `RegionFolder`, monomorphizing its callback and accounting for flags to avoid folding unnecessarily.

Author: bors