Auto merge of #54778 - scottmcm:stabilize-ihle, r=pnkfelix

Auto merge of #54778 - scottmcm:stabilize-ihle, r=pnkfelix

Stabilize impl_header_lifetime_elision in 2015

~~This is currently blocked on https://github.com/rust-lang/rust/issues/54902; it should be good after that~~

It's already stable in 2018; this finishes the stabilization.

FCP completed (https://github.com/rust-lang/rust/issues/15872#issuecomment-417953153), proposal (https://github.com/rust-lang/rust/issues/15872#issuecomment-412759783).

Tracking issue: https://github.com/rust-lang/rust/issues/15872
Usage examples (from libcore): https://github.com/rust-lang/rust/pull/54687