[MSVC] Support for __unaligned qualifier in functions

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270834 91177308-0d34-0410-b5e6-96231b3b80d8