Requires adding path segments to the hir map

Suggest using `'static` when a lifetime is missing in the return type
with a structured suggestion instead of a note.

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

This commit adds an `ImplicitSelfKind` to the HIR and the MIR that keeps
track of whether a implicit self argument is immutable by-value, mutable
by-value, immutable reference or mutable reference so that the addition
of the `mut` keyword can be suggested for the immutable by-value case.

This seemed like a good way to kick the tires on the
elided-lifetimes-in-paths lint (#52069)—seems to work! This was also
pretty tedious—it sure would be nice if `cargo fix` worked on this
codebase (#53896)!

(While still prohibiting explicit and in-band in the same header.)

(Not `Try` since `QuestionMark` is using that.)

The existing elided-lifetimes-in-paths lint (introduced in Nov. 2017's
accd997b / #46254) lacked stuctured suggestions and—much more
alarmingly—produced false positives on associated functions (like
`Ref::clone`) and on anonymous '_ lifetimes (!!—yes, the very
anonymous lifetimes that we meant to suggest "instead"). That this
went apparently unnoticed for so long maybe tells you something about
how many people actually bother to flip on allow-by-default lints.

After many hours of good old-fashioned American elbow grease—and a
little help from expert reviewers—it turns out that getting the right
answer is a lot easier if we fire the lint while lowering the Higher
Intermediate Representation.

The lint is promoted to the idioms-2018 group.

Also, in the matter of test filenames, "elided" only has one 'l' (see,
e.g., https://en.wiktionary.org/wiki/elide).

Resolves #52041.