Inline struct/tuple returns

Currently, you can only make normal structs that can be returned through RLS and not normal returns.
We should add declarations for inline structs which can be returned through (multiple) registers but not RLS:
```zig
const T = struct inline {
  a: u64,
  b: u64,
};

fn f() T {
  return .{
    .a = 5,
    .b = 3,
  };
}
```
->
```asm
f:
  mov rax, 5
  mov rbx, 3
  ret
```