clippy_dev parsing revamp part 6/N

[Jarcho]

Based on #15979

This part keeps all the parsed source files in memory. These are then borrowed from to avoid allocations and reused when generating which avoids reloading the files. The next PR will also use this when reporting errors.

changelog: none

[rustbot]

Some changes occurred in clippy_lints/src/doc

cc @notriddle

[rustbot]

r? @llogiq

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.