Use higher level types to represent classes

Nobody has claimed this yet.

Instead of using `LLVMStructType`, use a higher level Polygeist-specific type to represent classes holding more information (in addition to member), e.g., class name.

This would allow for easier handling in other dialects depending on Polygeist, e.g., conversion to dialect-specific types; without including further modifications in the Polygeist code.

E.g.,

```cpp
namespace ns {
namespace impl {
class A { int m; };
}
class B { A n; };
}
```

could be represented as:

```MLIR

>
```

This would allow a dialect Foo to represent the `B` class as, e.g.:

```MLIR

```

just by reading the namespace-qualified type name.,

No files, tests, or entry points are named. Start by locating Polygeist's uses of LLVMStructType and the existing type definitions, then trace how class names and members are represented across dependent dialects. Done means a Polygeist-specific class type preserves the qualified name and member information while allowing other dialects to consume it without further Polygeist changes.

Written by the indexing model from the issue text.