perf: avoid cloning associated items in empty_line_after

[xmakro]

empty_line_after's check_impl_item and check_trait_item converted every associated item into an ItemKind with item.kind.clone().into(), only to read its description string. The clone deep-copies the whole associated item, including function bodies, for each impl and trait item.

This reads the description directly from the AssocItemKind and passes the module's first-item id explicitly, so no ItemKind is constructed.

As part of this, I refactored the stored item kind from a description string into a small enum (ItemKindDescr). This is to avoid silent drift of str labels causing issues later on.

Instruction counts via valgrind --tool=callgrind (sum of clippy-driver Ir over a workspace re-lint), master vs this change:

crate	master	this PR	change
serde	3,361,589,306	3,312,393,958	-1.46%
syn	2,115,732,491	2,106,118,839	-0.45%
wasmi	12,471,489,259	12,419,942,932	-0.41%
ripgrep	1,319,851,434	1,314,547,652	-0.40%
regex	816,516,564	814,654,422	-0.23%

changelog: none

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[Jarcho]

Thank you for finding/fixing various perf issues.

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