[clang] [APINotes] Support nested tags (PR #99655)

[Egor Zhdan via cfe-commits]

================
@@ -456,6 +458,7 @@ template <> struct MappingTraits<Tag> {
     IO.mapOptional("EnumKind", T.EnumConvenienceKind);
     IO.mapOptional("SwiftCopyable", T.SwiftCopyable);
     IO.mapOptional("Methods", T.Methods);
+    IO.mapOptional("Tags", T.Tags);
----------------
egorzhdan wrote:

There are actually child tags, in the sense of tags declared within other tags, e.g.
```cpp
struct Outer {
  struct Inner {
  };
};
```

https://github.com/llvm/llvm-project/pull/99655