[clang] [clang] Improve heuristic resolution of template parameters reached indirectly (PR #223667)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 requested changes to this pull request.

Thanks for improving HeuristicResolver :)

I realized while reviewing this patch that the existing default template argument heuristic is a bit over-eager. For example:

```c++
struct Default {
  void foo();
};

struct Custom {
  void foo();
};

template <class T, class A = Default>
struct S {
  A bar();
};

template <class T>
void baz() {
  S<T, Custom> s;
  s.bar().foo();  // incorrectly targets Default::foo rather than Custom::foo
}
```

This patch will make that over-eagerness visible in more contexts, for example:

```c++
struct Default {
  void foo();
};

struct Custom {
  void foo();
};

template <class T, class A = Default>
struct S {
  typedef A type;
};

template <class T>
void baz() {
  typename S<T, Custom>::type t;
  t.foo();  // previously didn't resolve, now incorrectly resolves to Default::foo
}
```

Making these examples have the desired behaviour would be a more involved change, so no need to block on that now, but let's maybe add the above examples to the test suite with FIXMEs.

https://github.com/llvm/llvm-project/pull/223667