My experience is that while LLVM is very modular, it also has a pretty high amount of change in the boundaries, both in where they're drawn and in the interfaces between them. Maintaining a fork of LLVM with a new back-end is very hard.
Do you know why it's a fork? Als, from this https://github.com/ROCm/llvm-project/commits/amd-staging/ it looks like it might be more appropriately called a staging branch than a fork.
Various reasons, like embargoes on information, stuff we didn't want to wait for review on before shipping, or features that don't make sense for upstream like `hipcc` which is an `nvcc` wrapper.
Our goal is to get most modifications not in the third category into upstream at some point which makes the maintenance load bearable.
I know my company (AMD) maintains an llvm fork for ROCm. YMMV.