MS ABI: Consider alignment attributes on typedefs for layout

The MS ABI has a notion of 'required alignment' for fields; this
alignment supercedes pragma pack directives.

MSVC takes into account alignment attributes on typedefs when
determining whether or not a field has a certain required alignment.

Do the same in clang by tracking whether or not we saw such an attribute
when calculating the type's bitwidth and alignment.

This fixes PR20418.

Reviewers: rnk

Differential Revision: http://reviews.llvm.org/D4714

llvm-svn: 214274