There is a similar project for the Game Boy (sm83 cpu) with a fork of LLVM.
https://github.com/DaveDuck321/gb-llvm
https://github.com/DaveDuck321/libgbxx
It seems to be first reasonably successful attempt (can actually be used) among a handful of previous abandoned llvm Game Boy attempts.
The CE-dev community's LLVM back-end for the (e)Z80 'panned out' in that it produced pretty decent Z80 assembly code, but like most hobby-level back-ends the effort to keep up to date with LLVM's changes overwhelmed the few contributors active and it's now three years since the last release. It still works, so long as you're OK using the older LLVM (and clang).
This is why these back-ends aren't accepted by the LLVM project: without a significant commitment to supporting them, they're a liability for LLVM.
Most attempts at developing new LLVM downstream architectures simply fail at keeping up with upstream LLVM, especially across major releases. Perhaps these projects should focus a bit more on getting at least some of their simpler self-contained changes to be adopted upstream, such as custom optimization passes. Once that is done successfully, it might be easier to make an argument for also including support for a newly added ISA, especially a well-known ISA that can act as convenient reference code for the project as a whole.
Presumably it would be straightforward to port the GB code generation to the Intel 8080 / Z80. There have been a few attempts for LLVM for those CPUs over the years. But none which panned out, I think?