[MSVC] Support for __unaligned qualifier in functions

This implements support for MS-specific __unaligned qualifier in functions and
makes the following test case both compile and mangle correctly:

struct S {
    void f() __unaligned;
};
void S::f() __unaligned {
}

Differential Revision: http://reviews.llvm.org/D20437

llvm-svn: 270834