Auto merge of #16477 - davidsemakula:trait-incorrect-case-diagnostic, r=Veykril

feat: Add incorrect case diagnostics for traits and their associated items

Updates incorrect case diagnostic to:
- Check traits and their associated items
- Ignore trait implementations except for patterns in associated function bodies

Also cleans up `hir-ty::diagnostics::decl_check` a bit (mostly to make it a bit more DRY and easier to maintain)

Also fixes: #8675 and fixes: #8225

Author: bors