[clang] [Clang]fix dependent qualified base member lookup crash (PR #199403)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

Is this simply because we haven't set the DefinitionData's Definition member yet before calling `bases`?

I'd try to fix that race condition instead. The reason `bases` depends on the definition is a bit silly really, it just needs to get the ASTContext and any declaration in the program would do here.

https://github.com/llvm/llvm-project/pull/199403