Auto merge of #17814 - ShoyuVanilla:object-safety, r=Veykril

feat: Implement object safety and its hovering hint

Resolves #17779

- [x] Fill missing implementations
- [x] Hover rendering
- [x] Implement object safety's own test suite, like layout
- [x] Add test cases (from rustc maybe)
- [x] Clean up ugly codes
- [x] Add doc string

Author: bors