Auto merge of #42482 - eddyb:issue-42467, r=nikomatsakis

rustc: T: 'empty always holds for all types.

Fixes #42467 by special-casing `ReEmpty` to always hold, even for parameters.
The reason this is the case is that `ReEmpty` is the result of inferring a region variable with no constraints attached to it, so there is no lifetime a type would contain which would be strictly shorter.

r? @nikomatsakis

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