Remove dependancy on __refstring header; use local copy instead.

This patch removes the dependancy on libc++'s __refstring header,
which was only a header in the first place so that libc++abi could
build library code using it, and not because libc++ needed it in
the headers.

This patch allows libc++ to stop shipping <__refstring> publicaly
at the cost of duplicating it across projects. Ideally libc++abi
would always require the libc++ sources when building, but that's
a separate discussion I plan to start on the mailing lists shortly.

llvm-svn: 307748