[clang][ExtractAPI] Stop dropping fields of nested anonymous record types when they aren't attached to variable declaration (#104600)

- Introduce primitives for removing records from `APISet` and managing
the record chain of `RecordContext`
- Detect nested anonymous record types and remove them from the `APISet`
after they have been fully traversed and transfer ownership of child
records to the parent context (if any)

clang/lib/ExtractAPI/Serialization/SymbolGraphSerializer.cpp

@@ -673,14 +673,6 @@ bool SymbolGraphSerializer::shouldSkip(const APIRecord *Record) const {
   if (Record->Availability.isUnconditionallyUnavailable())
     return true;
 
-  // Filter out symbols without a name as we can generate correct symbol graphs
-  // for them. In practice these are anonymous record types that aren't attached
-  // to a declaration.
-  if (auto *Tag = dyn_cast<TagRecord>(Record)) {
-    if (Tag->IsEmbeddedInVarDeclarator)
-      return true;
-  }
-
   // Filter out symbols prefixed with an underscored as they are understood to
   // be symbols clients should not use.
   if (Record->Name.starts_with("_"))