MS ABI: vptr injection should obey alignment requirements

vptr injection must inject padding equivalent to the alignment of the
most aligned non-virtual subobject, not the alignment of the enclosing
record.

To fascilitate this change, don't let record layout observe the
alignment of the record until we've injected our vptrs. Also, do not
allow the alignment of vbases to affect required alignment until just
before we insert the vtordisp field.