CodeGen: Don't crash when initializing pointer-to-member fields in bases

Clang uses two types to talk about a C++ class, the
NonVirtualBaseLLVMType and the LLVMType.  Previously, we would allow one
of these to be packed and the other not.

This is problematic.  If both don't agree on a common subset of fields,
then routines like getLLVMFieldNo will point to the wrong field.  Solve
this by copying the 'packed'-ness of the complete type to the
non-virtual subobject.  For this to work, we need to take into account
the non-virtual subobject's size and alignment when we are computing the
layout of the complete object.

This fixes PR21089.