[clang] class template deduction guides is not accepting type alias to to the template itself

clang is only C++ compiler not accepting following code:

```c++
template struct tmpl_type;

using Integer = tmpl_type;

template struct tmpl_type { };

tmpl_type(int) -> Integer;
```
https://compiler-explorer.com/z/PsdGefz5h

It gives confusing error message:
> deduced type 'Integer' (aka 'tmpl_type') of deduction guide is not a specialization of template 'tmpl_type'

I know that the standard mandates simple-template-id there [temp.deduct.guide#1](https://eel.is/c++draft/temp.deduct.guide#1), but I think it should be an extension to allow alias leading to same template for which the CTAD is. Especially because clang has attribute `[[clang::preferred_name(NAME)]]`.

Start by compiling the minimal C++ reproducer from the issue and compare the behavior with [temp.deduct.guide#1]. Read the discussion about allowing an alias to the same template, including the relationship to clang::preferred_name; done means the intended behavior and diagnostic are agreed and covered by compiler tests.

Written by the indexing model from the issue text.