Rollup merge of #70913 - eddyb:rc-arc-diagnostic-items, r=matthewjasper

Replace "rc"/"arc" lang items with Rc/Arc diagnostic items.

`Rc`/`Arc` should have no special semantics, so it seems appropriate for them to not be lang items.

r? @matthewjasper

diff --cc src/librustc_middle/ty/context.rs
Simple merge

diff --cc src/librustc_middle/ty/mod.rs
Simple merge

diff --cc src/librustc_middle/ty/sty.rs
Simple merge