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
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: e3e7c0fb37adc8e3bd92fe550ca444edc0b75a75
diff --git a/src/stdexcept.cpp b/src/stdexcept.cpp
index 3f33330..5e06e52 100644
--- a/src/stdexcept.cpp
+++ b/src/stdexcept.cpp
@@ -11,7 +11,7 @@
 #include "new"
 #include "string"
 #include "system_error"
-#include "__refstring"
+#include "include/refstring.h"
 
 /* For _LIBCPPABI_VERSION */
 #if !defined(_LIBCPP_BUILDING_HAS_NO_ABI_LIBRARY) && \