Remove <__refstring> header; Move it into source directory.

The libc++ <__refstring> headers has no real reason why it should
be a public header that libc++ ships. The only reason it was in the include
directory was because libc++abi needed it to build the library.

However keeping <__refstring> a header had other problems, like requiring its
dependancies to also be in the headers. For that reason this patch
moves it into the source directory.

To work around libc++abi's need for this header a duplicated copy was added
to libc++abi in r307748. While duplicating the code is an unfortunate solution
it's the best solution that's currently possible.

In the future I would like to start a discussion on the mailing lists about
making libc++abi build as a sub-project of libc++, requiring the libc++ sources
always be present.

llvm-svn: 307749