[MS-ABI] Drop Special Layout in 64-bit mode.

As of cl.exe version 18, the special layout rules for structs with 
alignment 16 or greater has been dropped.  This patch drops the behavior 
from clang.  This patch also updates the lit tests to reflect the 
change.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204674 91177308-0d34-0410-b5e6-96231b3b80d8