[clang] [clang] Fix missing diagnostic of declaration use when accessing TypeDecls through typename access (PR #129681)

[Hans Wennborg via cfe-commits]

zmodem wrote:

We're seeing new -Wunguarded-availability warnings after this. A reduced example:

```
$ cat /tmp/x.cc
template<class T> struct remove_cv { typedef T type; };

struct __attribute__((__availability__(android, introduced = 29))) AAudioStreamWrapper { };

template <typename T> struct Foo {
  T *p;
};

template <typename T> struct Bar {
  typename remove_cv<T>::type *p;
};


struct __attribute__((__availability__(android, introduced = 29))) User {
  AAudioStreamWrapper a; // okay
  Foo<AAudioStreamWrapper> b; // okay

  Bar<AAudioStreamWrapper> c; // not okay?
};

$ build/bin/clang -c -target aarch64-linux-android26 -Wunguarded-availability /tmp/x.cc
/tmp/x.cc:10:26: warning: 'AAudioStreamWrapper' is only available on Android 29 or newer [-Wunguarded-availability]
   10 |   typename remove_cv<T>::type *p;
      |                          ^
/tmp/x.cc:18:28: note: in instantiation of template class 'Bar<AAudioStreamWrapper>' requested here
   18 |   Bar<AAudioStreamWrapper> c; // not okay?
      |                            ^
/tmp/x.cc:3:68: note: 'AAudioStreamWrapper' has been marked as being introduced in Android 29 here, but the deployment target is Android 26
    3 | struct __attribute__((__availability__(android, introduced = 29))) AAudioStreamWrapper { };
      |                                                                    ^
/tmp/x.cc:9:30: note: annotate 'Bar<AAudioStreamWrapper>' with an availability attribute to silence this warning
    9 | template <typename T> struct Bar {
      |                              ^
1 warning generated.
```

I believe the new warning is not correct, since `c` (like the other members) is in fact guarded by the availability attribute.

I'll revert to green for now until this can be investigated.

https://github.com/llvm/llvm-project/pull/129681