[clang] [clang] Fix mangling of constrained auto referring to a parameter when the return type has an ABI tag (PR #223121)

[Younan Zhang via cfe-commits]

================
@@ -648,6 +645,12 @@ features cannot lower the translation-unit ABI level;
   to a subobject and is used in a context that requires an implicit conversion.
   (#GH215900)
 
+- Fixed an assertion when mangling an abbreviated function template whose
+  constrained `auto` parameter refers to an earlier parameter (e.g.
+  `template<typename T> auto f(T t, C<decltype(t)> auto) -> S`) and whose
+  return type has an ABI tag, such as a type declared in an `abi_tag` inline
+  namespace like `std::string` under libstdc++. (#GH204178)
+
----------------
zyn0217 wrote:

```suggestion
- Fixed an assertion when mangling an abbreviated function template whose
  return type has an ABI tag (#GH204178)
```

https://github.com/llvm/llvm-project/pull/223121