It turns out that LLD caching hasn't been in use for a while. On master,
it is currently only enabled when you compile via the build system,
passing `-fincremental`, using LLD (and so LLVM if there's a ZCU). That
case never happens, because `-fincremental` is only useful when you're
using a backend *other* than the LLVM backend. My previous commits
accidentally re-enabled this logic in some cases, exposing bugs; that
ultimately led to this realisation. So, let's just delete that logic --
less LLVM-related cruft to maintain.