[clang] Reapply "[clang] Fix name lookup for dependent bases" (PR #118003)

[Mike Hommey via cfe-commits]

glandium wrote:

I was creducing it (well, cvise-ing). Here's a reproducer:
```
template <typename, typename, bool> struct MozPromise {
  class Private;

private:
  void *mMagic4;
};
template <typename ResolveValueT, typename RejectValueT, bool IsExclusive>
struct MozPromise<ResolveValueT, RejectValueT, IsExclusive>::Private
    : MozPromise {
  void SetTaskPriority() { mMagic4 }
}
```

https://github.com/llvm/llvm-project/pull/118003