[clang] [KeyInstr][Clang] Ret atom (PR #134652)

[Orlando Cazalet-Hyams via cfe-commits]

https://github.com/OCHyams commented:

> If I understand correctly, most of the test changes are to ensure that return instructions get /an/ atom group, without being specific about which? IMO it'd be better to make that number explicit to avoid a return accidentally being part of the wrong atom group. It's necessary for it to be a distinct group, right? (A chore, but an important fact).

Done. Rebased (to address an inline comment and capture since-merged tests).

https://github.com/llvm/llvm-project/pull/134652