[MS ABI] Implement a mangling for _Atomic types

[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>)'

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@256557 91177308-0d34-0410-b5e6-96231b3b80d8