__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.
As an alternative, we could continue not mangling this qualifier. But if I read the code right, in order to get substitutions correctly, we would have to rebuild some types and stripe the __unaligned qualifier prior any mangling and substitution.
This patch also removes the change done in D31976 and updates its test to the new reality.