[ms-abi] Change the way alignment is tracked

This patch more cleanly seperates the concepts of Preferred Alignment 
and Required Alignment.  Most notable that changes to Required Alignment 
do *not* impact preferred alignment until late in struct layout.  This 
is observable when using pragma pack and non-virtual bases and the use 
of tail padding when laying them out.

Test cases included.

llvm-svn: 198988