Revert "[gn build] Don't set LLVM_UNREACHABLE_OPTIMIZE when llvm_enable_assertions"

This reverts commit 0f19c603.

This didn't actually do anything. llvm_unreachable() under `#ifndef NDEBUG` is always supposed to report an error regardless of LLVM_UNREACHABLE_OPTIMIZE. I can't reproduce the issue I was originally seeing with this reverted, not sure what was happening back then, manually verified by messing around with various binaries/configurations.