[clang] [Clang] Protect ObjCMethodList assignment operator against self-assignment (PR #97933)

[via cfe-commits]

smanna12 wrote:

> As far as I can tell, `ObjCMethodList` is already self-assignment safe so long as `PointerIntPair` is. It looks like the latter might not be though since its copy assignment operator (as implemented via `PunnedPointer`) uses `memcpy` and it does not guard against self-assignment. I wonder if a change should instead be made there. Can you share more details from the static analysis report?

Thank you @tahonermann for reviews and feedbacks.

This is what static analyzer tool complains. It does not provide much detailes:
```
In clang::​ObjCMethodList::​operator =(clang::​ObjCMethodList const &): Assignment operator does not check for self-assignment
 ObjCMethodList &operator=(const ObjCMethodList &L) {
  	
 //Unsafe assignment operator (SELF_ASSIGN)
self_assign: No protection against the object assigning to itself.
    MethodAndHasMoreThanOneDecl = L.MethodAndHasMoreThanOneDecl;
    NextAndExtraBits = L.NextAndExtraBits;
    return *this;
  }

https://github.com/llvm/llvm-project/pull/97933