(new) Compiler for (old) XCode
XCode 4.2 will run on Snow Leopard and it has ARC support.

Go to Apple Developer Downloads and search for XCode 4.2 for Snow Leopard, it will show up there, at least if you're a registered iOS developer.

Any newer XCode version than 4.2 is not supported by Apple on Snow Leopard, so I suspect you may have problems even if you'd manage to install it.