Auto merge of #23452 - nikomatsakis:unsafety-subtyping, r=nrc

Safe fns are no longer subtypes of unsafe fns, but you can coerce from one to the other.

This is a [breaking-change] in that impl fns must now be declared `unsafe` if the trait is declared `unsafe`. In some rare cases, the subtyping change may also direct affect you, but no such cases were encountered in practice.

Fixes #23449.

r? @nrc

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

diff --cc src/librustc_trans/trans/consts.rs
Simple merge

diff --cc src/librustc_trans/trans/expr.rs
Simple merge

diff --cc src/librustc_typeck/check/mod.rs
Simple merge