[clang][Interp] Fix derived-to-base casts for >1 levels

[tbaeder]

Summary

The GetPtrBasePop op we were using only works for direct base classes.

[tbaeder]

Ping

[aaron.ballman]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

1956

clang/test/AST/Interp/records.cpp

278

I think it'd be good to add test coverage for cases like: https://godbolt.org/z/GnPnP4z76

[tbaeder]

clang/test/AST/Interp/records.cpp

278

That even worked :)

[aaron.ballman]

LGTM!