Update `repr` to use the attribute template

[ehuss]

New rules:

• ❗ layout.repr.attribute.intro
• ❗ layout.repr.attribute.syntax
• ❗ layout.repr.attribute.allowed-positions
• ❗ layout.repr.attribute.duplicates
• ❗ layout.repr.attribute.combinations
• ❗ layout.repr.attribute.combinations.transparent
• ❗ layout.repr.attribute.combinations.rust
• ❗ layout.repr.attribute.combinations.primitive
• ❗ layout.repr.attribute.combinations.primitive-c
• ❗ layout.repr.attribute.combinations.primitive-unit-only
• ❗ layout.repr.attribute.combinations.align-packed
• ❗ layout.repr.attribute.combinations.align-packed-rust-or-c
• ❗ layout.repr.attribute.combinations.align-max
• ❗ layout.repr.attribute.combinations.packed-min
• ❗ layout.repr.attribute.combinations.rust-c-dupe

Renamed rules:

• layout.repr.align-packed is now layout.repr.alignment.intro
• layout.repr.primitive.constraint split into layout.repr.primitive.enum-only and layout.repr.primitive.zero-variant
• layout.repr.alignment.enum is now layout.repr.alignment.align-enum
• layout.repr.alignment.constraint-exclusive split into layout.repr.alignment.pack-transitive-aligned and layout.repr.alignment.packed-enum
• layout.repr.transparent.constraint-exclusive moved to layout.repr.attribute.combinations

[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.