[clang] [ObjC] Defer to the LLVM backend for unaligned atomic load and stores (PR #79191)

[via cfe-commits]

AtariDreams wrote:

> Does this cause an ABI incompatibility? E.g. if we have a case where an existing object calls copyStruct on a given object (which presumably has its own internal mutex), and a newly compiled object file calls `__atomic_load` on the same object, implemented with its own internal mutex, the operations won't be atomic w.r.t. each-other.
> 
> I don't know enough about ObjC codegen stuff to be able to say if there are mitigating factors that make this OK (e.g. if it's all within one TU?).
> 
> Sidenote: please avoid rebase, amend, and force-pushing, because it makes it hard to review what's been changed -- instead, just keep adding more commits to your pull-request branch.

Don't know but everything passed now!

https://github.com/llvm/llvm-project/pull/79191