The modified test would just crash without the code change. The reason is that
we would try to extend the Foo type imported from the PCH debug info when adding the
Foo::Bar definitiion to it. This will crash if the type is not complete.
I pondered moving the CompleteType call inside of CopyType, but CopytType seems
to be used as a lower-level building block in other places so I decided not to.
ClangASTImporter is kinda scary. It has no comments and interacts with the Clang
ASTs which are not exactly easy to deal with. Any insight appreciated.