[clang] [clang][Sema] Use original template pattern when declaring implicit deduction guides for nested template classes (PR #68379)

[via cfe-commits]

antangelo wrote:

I've come up with this minimal reproducer:

```cpp
template<class A>
struct outer {
    template<class B>
    struct inner {
        inner(B b);
        inner(const inner &x) = default;
    };

    template<class B>
    inner(B b) -> inner<B>;
};

outer<int>::inner i(1);
```

The error seems to be triggered by the inclusion of both the copy constructor and the explicit deduction guide. I'll revert the patch now until I can come up with a proper fix for it.

https://github.com/llvm/llvm-project/pull/68379