[Clang][Sema] Substitute for the type aliases inside of a CTAD guide (#94740)
Similar to the approach of handling nested class templates when building
a CTAD guide, we substitute the template parameters of a type alias
declaration with the instantiating template arguments in order to ensure
the guide eventually doesn't reference any outer template parameters.
For example,
```cpp
template <class T> struct Outer {
using Alias = S<T>;
template <class U> struct Inner {
Inner(Alias);
};
};
```
we used to retain the reference to T accidentally because the
TreeTransform does nothing on type alias Decls by default.
Fixes https://github.com/llvm/llvm-project/issues/94614
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