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

[Orlando Cazalet-Hyams via cfe-commits]

https://github.com/OCHyams commented:

Ready for review

* Add parameter `RetKeyInstructionsSourceAtom` in `EmitFunctionEpilog` to request a `ret` gets a specific atomGroup number through `addInstToSpecificSourceAtom`

Most of the updated tests just check for any atom on the ret, but the two tests added in this PR check for specific atom numbers.

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