[clang] [SystemZ][z/OS] Implement z/OS XPLINK ABI (PR #91384)

[Kai Nacke via cfe-commits]

https://github.com/redstar commented:

Sorry, I left some more comments.
In addition, there is no test involving a union. That is especially interesting for the complex-like structures. E.g.
```
union Float2 {
  float a;
  float b;
};

struct Elem {
  float a;
  Float2 b;
};

Elem calc(Elem);
```
A test case involving vector types is also missing.

https://github.com/llvm/llvm-project/pull/91384