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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@307749 91177308-0d34-0410-b5e6-96231b3b80d8

src/include/refstring.h

@@ -7,8 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef _LIBCPP___REFSTRING
-#define _LIBCPP___REFSTRING
+#ifndef _LIBCPP_REFSTRING_H
+#define _LIBCPP_REFSTRING_H
 
 #include <__config>
 #include <stdexcept>
@@ -124,4 +124,4 @@ bool __libcpp_refstring::__uses_refcount() const {
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif //_LIBCPP___REFSTRING
+#endif //_LIBCPP_REFSTRING_H

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) && \