Auto merge of #8289 - jubnzv:unspecified-layout-union, r=camsteffen

Add `default_union_representation` lint

Closes #8235

changelog: Added a new lint  [`default_union_representation`]

diff --cc CHANGELOG.md
Simple merge

diff --cc clippy_lints/src/lib.register_lints.rs
Simple merge

diff --cc clippy_lints/src/lib.rs
Simple merge