OK. I was poking around using the '-v' option to 'clang' to try and work out which linker was actually
being used. It pointed me at the right toolchain folder, where I found the magic command 'dsymutil'.
This looked interesting, especially when I checked out the man page.
So I ran it, and got a huge folder full of output that seems to give lldb what it needs.
So for non-Mac types (like me)...
You need to run a post-link step... Simply run 'dsymutil' on the final executable (or dylib). It should
build a folder ('bundle' in Mac-speak) with the debug symbol data in it.
I wasn't sure where to put the resulting XXXX.dSym folder on the target machine, but simply copying
it alongside the binary seems to do the job.
Personally, I'd prefer to keep debug symbol data embedded (less chance of mislaying it). But I'm happy
as long as the debugger works!