[clang] [KeyInstr][Clang] Add ApplyAtomGroup (PR #134632)

[Orlando Cazalet-Hyams via cfe-commits]

OCHyams wrote:

I jumped forward to the `ret` patch and found a simpler solution. I haven't updated the `ret` pull request just yet (will get to it soon), but it basically just involves passing an atom group number through a single specific function to pass to `addInstToSpecificSourceAtom`.

How does this look now?

https://github.com/llvm/llvm-project/pull/134632