[clang] [Clang] add user-level sizeless attribute (PR #71894)

[Timm Baeder via cfe-commits]

================
@@ -2400,7 +2402,66 @@ bool Type::isWebAssemblyTableType() const {
   return false;
 }
 
-bool Type::isSizelessType() const { return isSizelessBuiltinType(); }
+bool Type::isSizelessType() const {
+  // Check if this type or any of its constituents are sizeless, due to
+  // being a builtin type or individually having the user attribute.
+  // As structs can be recursive, we iterate through without repeats.
+  SmallVector<const Type *, 1> todo = {this};
+  llvm::SmallPtrSet<const Type *, 1> done;
----------------
tbaederr wrote:

```suggestion
  llvm::SmallPtrSet<const Type *, 1> Done;
```

https://github.com/llvm/llvm-project/pull/71894