How do I add AVR support for Clang?

Currently LLVM

has no target AVR

and this fork is for adding an AVR backend / target. It seems to be ready to compile the LLVM bitcode into an AVR assembly. But Clang

does not support AVR target (it does not support AVR attributes like "progmem" and data layout). How can I add AVR support for Clang? I need a guide like this and I understand that I need AVR knowledge.