Avoid assert when a non-static member function is qualified with __unaligned

Under -fms-extensions __unaligned is a type-qualifier that can be applied to a
non-static member function declaration.

This causes an assertion when mangling the name under Itanium, where that
qualifier is not mangled.

This patch justs makes the minimal change to avoid the crash and avoid mangling
__unaligned, as it currently happens with non-member functions.

Differential Revision: https://reviews.llvm.org/D31976

llvm-svn: 300686