Make libc++'s versioning namespace customizable

Summary:
This patch makes the versioning namespace libc++ uses customizable by the user using `-DLIBCXX_ABI_NAMESPACE=__foo`. 

This allows users to build custom versions of libc++ which can be linked into binaries with other libc++ versions without causing symbol conflicts or ODR issues. 

Reviewers: mclow.lists, ldionne

Reviewed By: ldionne

Subscribers: kristina, smeenai, mgorny, phosek, libcxx-commits

Differential Revision: https://reviews.llvm.org/D53879

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345657 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -119,7 +119,9 @@
 #define _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_X##_LIBCPP_Y
 #define _LIBCPP_CONCAT(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y)
 
-#define _LIBCPP_NAMESPACE _LIBCPP_CONCAT(__,_LIBCPP_ABI_VERSION)
+#ifndef _LIBCPP_ABI_NAMESPACE
+# define _LIBCPP_ABI_NAMESPACE _LIBCPP_CONCAT(__,_LIBCPP_ABI_VERSION)
+#endif
 
 #if __cplusplus < 201103L
 #define _LIBCPP_CXX03_LANG
@@ -791,9 +793,9 @@ typedef __char32_t char32_t;
 #define _LIBCPP_INLINE_VISIBILITY _LIBCPP_HIDE_FROM_ABI
 
 // Inline namespaces are available in Clang/GCC/MSVC regardless of C++ dialect.
-#define _LIBCPP_BEGIN_NAMESPACE_STD namespace std { inline namespace _LIBCPP_NAMESPACE {
+#define _LIBCPP_BEGIN_NAMESPACE_STD namespace std { inline namespace _LIBCPP_ABI_NAMESPACE {
 #define _LIBCPP_END_NAMESPACE_STD  } }
-#define _VSTD std::_LIBCPP_NAMESPACE
+#define _VSTD std::_LIBCPP_ABI_NAMESPACE
 _LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 17

include/__config_site.in

@@ -28,6 +28,7 @@
 #cmakedefine _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 #cmakedefine _LIBCPP_NO_VCRUNTIME
+#cmakedefine _LIBCPP_ABI_NAMESPACE @_LIBCPP_ABI_NAMESPACE@
 
 @_LIBCPP_ABI_DEFINES@