[ms-abi] Change the way alignment is tracked

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198988 91177308-0d34-0410-b5e6-96231b3b80d8