[MSVC] Implementation of __unaligned as a proper type qualifier

This patch implements __unaligned (MS extension) as a proper type qualifier
(before that, it was implemented as an ignored attribute).

It also fixes PR27367 and PR27666.

Differential Revision: http://reviews.llvm.org/D20103


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269220 91177308-0d34-0410-b5e6-96231b3b80d8