Auto merge of #23691 - richo:dedup-typeorigin-mergable, r=eddyb

I've started on refactoring the error handling code to avoid the need to reparse generated errors in `span_*`, but would rather land this incrementally as one monolithic PR (and have un-fond memories of merge conflicts from various other monoliths)

r? @eddyb

diff --cc src/librustc/middle/infer/mod.rs
Simple merge