Auto merge of #53016 - scottmcm:impl-header-lifetime-elision, r=nikomatsakis

Extract impl_header_lifetime_elision out of in_band_lifetimes

This way we can experiment with `impl Debug for &MyType` separately from `impl Debug for &'a MyType`.

I can't say I know what the code in here is doing, so please let me know if there's a better way :slightly_smiling_face:

I marked this as enabled in 2018 so that edition code continues to work without another flag.

Actual feature PR https://github.com/rust-lang/rust/pull/49251; Tracking Issue https://github.com/rust-lang/rust/issues/15872; In-band lifetimes tracking issue https://github.com/rust-lang/rust/issues/44524.

cc @aturon, per discussion on discord earlier
cc @cramertj & @nikomatsakis, who actually wrote these features