This correctly renders the vtable in SemIR, including allowing overrides
in
Carbon-derived-from-C++ classes.
It doesn't work in lowering because clang walks the methods of the
CXXRecordDecl - and we currently don't export anything into the
CXXRecordDecl's methods (we do export the fields) - so that's next.
This also doesn't teach Clang to affirmatively emit the vtable
regardless of the types use in C++ code - or to have Carbon use the
vtable in an object's initialization.