[Clang] Perform derived-to-base conversion on explicit object parameter in lambda (#89828)

Consider this code:
```c++
template <typename... Ts>
struct Overloaded : Ts... { using Ts::operator()...; };

template <typename... Ts>
Overloaded(Ts...) -> Overloaded<Ts...>;

void f() {
  int x;
  Overloaded o {
    [&](this auto& self) {
      return &x;
    }
  };
  o();
}
```

To access `x` in the lambda, we need to perform derived-to-base
conversion on `self` (since the type of `self` is not the lambda type,
but rather `Overloaded<(lambda type)>`). We were previously missing this
step, causing us to attempt to load the entire lambda (as the base
class, it would end up being the ‘field’ with index `0` here), which
would then assert later on in codegen.

Moreover, this is only valid in the first place if there is a unique and
publicly accessible cast path from the derived class to the lambda’s
type, so this also adds a check in Sema to diagnose problematic
cases.

This fixes #87210 and fixes #89541.