Language frontends are typically implemented as independent projects that merely use LLVM as a library. It is not necessary to create an LLVM fork for this purpose. You only link against it.
If you’re using the C++ API, which is unstable, you likely want to support only a narrow range of LLVM versions (or even require a specific one). If you’re using the C API, which is semi-stable, supporting a wider range of LLVM versions becomes easier.