introducing lint reason annotations (RFC 2383)

This is just for the `reason =` name-value meta-item; the
`#[expect(lint)]` attribute also described in the RFC is a problem for
another day.

The place where we were directly calling `emit()` on a match block
(whose arms returned a mutable reference to a diagnostic-builder) was
admittedly cute, but no longer plausibly natural after adding the
if-let to the end of the `LintSource::Node` arm.

This regards #54503.

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