Auto merge of #102903 - compiler-errors:region-var-leak, r=jackh726

Assert if inference vars are leaking from `InferCtxt::fully_resolve`

`InferCtxt::fully_resolve` shouldn't return unresolved inference vars without us at least being aware of it, so make it an assertion now. This should only happen in cases where we used to be returning `ReEmpty`...

cc `@jackh726`

Author: bors