Mark module atomic as cplusplus11.

With clang, the header atomic requires __has_feature(cxx_atomic), which is only
true in c++11 mode. Because of this, when using modules in c++98 with libc++
compilation of the std module would fail without this change, PR21002.

(With gcc, only gcc4.7+ is needed, no c++11. But gcc doesn't have modules yet,
and the module.modulemap language can't express things like "this is only
required if the compiler is clang". If gcc gets module support, we'd probably
have a module.modulemap file for each compiler that libc++ supports?)

llvm-svn: 218372