[RecordLayout] Fix ItaniumRecordLayoutBuilder so that is grabs the correct...

[RecordLayout] Fix ItaniumRecordLayoutBuilder so that is grabs the correct bases class offsets from the external source

Currently the ItaniumRecordLayoutBuilder when laying out base classes has the virtual
and non-virtual bases mixed up when pulling the base class layouts from the external source.

This came up in an LLDB bug where on arm64 because of differences in how it deals with
tail padding would layout the bases differently without the correct layout from the
external source (LLDB). This would result in some fields being off by 4 bytes.

Differential Revision: https://reviews.llvm.org/D83008