Add context to "const in pattern" errors
*Each commit addresses specific diagnostics.*
- Add primary span labels
- Point at `const` item, and `const` generic param definition
- Reword messages and notes
- Point at generic param through which an associated `const` is being referenced
- Silence const in pattern with evaluation errors when they come from `const` items that already emit a diagnostic
- On non-structural type in const used as pattern, point at the type that should derive `PartialEq`
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