auto merge of #17978 : arielb1/rust/remaining-garbage, r=nikomatsakis

it seems to be some kind of leftover GC-related detritus

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

diff --cc src/librustc/middle/mem_categorization.rs
Simple merge

diff --cc src/librustc/middle/typeck/check/regionck.rs
Simple merge