Mangle __unaligned in Itanium ABI

__unaligned is not currently mangled in any way in the Itanium ABI. This causes
failures when using -fms-extensions and C++ in targets using Itanium ABI.

As suggested by @rsmith the simplest thing to do here is actually mangle the
qualifier as a vendor extension.

This patch also removes the change done in D31976 and updates its test to the
new reality.

This fixes
  https://bugs.llvm.org/show_bug.cgi?id=33080
  https://bugs.llvm.org/show_bug.cgi?id=33178

Differential Revision: https://reviews.llvm.org/D33398

llvm-svn: 304523