[clang] Fix specialization of non-templated member classes of class templates

Explicit specialization doesn't increase depth of template parameters,
so need to be careful when gathering template parameters for
instantiation.
For the case:
```
template<typename T>
struct X {
  struct impl;
};

template <>
struct X<int>::impl {
    template<int ct>
    int f() { return ct; };
};
```
instantiation of `f` used to crash because type template parameter
`int` of explicit specialization was taken into account, but non-type
template parameter `ct` had zero depth and index so wrong parameter
ended up inside of a wrong handler.

Fixes https://github.com/llvm/llvm-project/issues/61159

Reviewed By: aaron.ballman, shafik

Differential Revision: https://reviews.llvm.org/D155705