[clang] [llvm] [Clang] Emit DW_TAG_template_alias for template aliases (PR #87623)

[Orlando Cazalet-Hyams via cfe-commits]

OCHyams wrote:

Thanks @dwblaikie 

> Bit unfortunate to store template parameters in different ways (in the extraData for the alias template, but in the templateParams for the composite types) - but I guess it'd be more invasive to try to represent alias templates as composite types?

I hadn't actually tried using DICompsiteType. I didn't choose DIDerivedType for any particularly principled reasons: typedefs use derived types which made it very easy to find places that needed updating, and an alias "felt" more like a derived type than composite type. Having looked only briefly, I don't _think_ it'd be any more invasive to use composite types instead (not 100% sure without diving in). I'm happy to give that a go if that is your preference?

---

I've just found an input that causes an assertion failure in `CollectTemplateParams` with my implementation:
```
template<typename Y, typename Z>
struct X {
  Y m1;
  Z m2;
};

template<typename... Ts>
using A = X<Ts...>;

A<int, int> a;
```

There's a mismatch between the template parameter list and template argument list sizes, created here:
```
  if (CGM.getCodeGenOpts().DebugTemplateAlias) {
    TemplateArgs Args = {TD->getTemplateParameters(), Ty->template_arguments()}; // <--- here
 ```
 
I notice that  we emit `DW_TAG_GNU_template_parameter_pack` for, e.g., variadic template struct instantiations. I've not figured out how to fix this just yet, but thought I'd bring it up in case there's something obvious.

https://github.com/llvm/llvm-project/pull/87623