SemaDeclCXX.cpp is where the Attrs should be attached to the base specifiers, and CXXBaseSpecifiers will have to be extended with functions and field to support adding and accessing attributes.
We decided to introduce instead the attribute base_arg("BaseClass", "Rpls") on the class itself since it is currently supported by clang and will not require modifications to clang's code.
SemaDeclCXX.cpp is where the Attrs should be attached to the base specifiers, and CXXBaseSpecifiers will have to be extended with functions and field to support adding and accessing attributes.