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