[libc++abi] Reuse libc++'s refstring.h header instead of copying it

This has been a long-standing TODO item, however we have now been requiring
a monorepo layout to build libc++ and libc++abi for a while now. Hence,
we can fix this code duplication issue now.

Note that it's still not super pretty to reach into libc++ to include
headers, but it's better than having duplicated code which can get out
of sync.