Use __config_site when building libc++. Also cleanup ABI versioning doc

llvm-svn: 250261
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 29ada6d178890d843133fd67300b96427951086a
diff --git a/docs/Abi.rst b/docs/DesignDocs/ABIVersioning.rst
similarity index 100%
rename from docs/Abi.rst
rename to docs/DesignDocs/ABIVersioning.rst
diff --git a/docs/index.rst b/docs/index.rst
index f34979f..7887611 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -128,6 +128,7 @@
    :maxdepth: 1
 
    DesignDocs/CapturingConfigInfo
+   DesignDocs/ABIVersioning
 
 
 * `<atomic> design <http://libcxx.llvm.org/atomic_design.html>`_