[clang] [Clang] Correctly deduce default argument for alias CTAD (PR #193836)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

@zyn0217 @mizvekov 

This does not actually fully solve the problem yet,

Given 

```cpp
template <auto>
struct A {};

template <class U>
using Foo = A<U{}>;

template <class UUUU = int>
using Bar = Foo<UUUU>;

Bar a{};
```

`UUUU` is correctly deduced, but not correctly substituted - we end up with `A<U{}>' (without substitution at runtime)
Which is a bit weird given that `transformTemplateParameter` should transform the argument.

Anyway, time for me to call it a night, I'll see tomorrow if either of you has a brilliant idea :)

(CTAD for alias is so complicated, not sure if it's the feature or our implementation of it...)


https://github.com/llvm/llvm-project/pull/193836