It's difficult because GCC is pretty buggy, which makes it hard to follow recent stable releases. In addition, OpenBSD has a lot of exploit mitigations in their branch of the compiler which will take considerable effort to port to LLVM.
This means they rely on "old" compilers to compile "old" C code from the base.
I.e. Chromium is build with clang (3.8) if I'm not mistaken.