[MS ABI] Implement a mangling for _Atomic types

MSVC doesn't implement a mangling for C11's _Atomic so we must invent
our own.

For now, treating it like a class type called _Atomic in the __clang
namespace.

This means that 'void f(__Atomic(int))'
will demangle as: 'void f(struct __clang::_Atomic<int>)'

llvm-svn: 256557