[Clang] Deducing this Lambda ignores assignment rules for captured references

[Link1J]

When using C++23 deducing this, captured references are assignable to anything.
The following code compiles with clang -std=c++23, when it clearly shouldn't. Godbolt

struct function {};
int main() {
    function list;
    [&list](this auto self) {
        list = self;
    }();
}

It even works with types that are not assignable. Godbolt

struct function {
    function& operator=(function const&) = delete;
};
int main() {
    function list;
    [&list](this auto self) {
        list = self;
    }();
}

And even it doesn't even need self as part of the assignment expression. Godbolt

struct function {
    function& operator=(function const&) = delete;
};
int main() {
    function list;
    [&list](this auto self) {
        list = function{};
    }();
}

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Jared Irwin (Link1J)

When using C++23 deducing this, captured references are assignable to anything. The following code compiles with `clang -std=c++23`, when it clearly shouldn't. [Godbolt](https://godbolt.org/z/7TMoPK6an) ```c++ struct function {}; int main() { function list; [&list](this auto self) { list = self; }(); } ``` It even works with types that are not assignable. [Godbolt](https://godbolt.org/z/scazq7sfo) ```c++ struct function { function& operator=(function const&) = delete; }; int main() { function list; [&list](this auto self) { list = self; }(); } ``` And even it doesn't even need `self` as part of the assignment expression. [Godbolt](https://godbolt.org/z/8jxs4Pxbb) ```c++ struct function { function& operator=(function const&) = delete; }; int main() { function list; [&list](this auto self) { list = function{}; }(); } ```

[Sirraide]

This is basically #84163 but also not technically a duplicate; either way, it’s also fixed by #84473.