This is a follow-up to d3e43bbe which resolved the issue for value types, but not for foreign reference types.
This change teaches IRGen that a base type of a C++ type might occupy less memory than its `sizeof`, specifically, it might only use `dsize` amount of memory.
rdar://147527755
In C++, a primary base class that is placed in the beginning of the type's memory layout isn't always the type that is the first in the list of bases – the base types might be laid out in memory in a different order.
This makes sure that IRGen handles base types of C++ structs in the correct order.
This fixes an assertion in asserts-enabled compilers, and an out-of-memory error in asserts-disabled compilers. The issue was happening for both value types and foreign reference types. This change also includes a small refactoring to reuse the logic between the two code paths.
rdar://140848603
