Add documentation and tests for Clangs C++11 extensions in C++03.

As we gear up to drop support for GCC in C++03, we should make clear
what our C++03 mode is, the C++11 extensions it provides,
and the C++11 extensions it depends on.

The section of this document discussing user-facing extensions has
been left blank while the community discusses new directions. For now
it's just a warning to users.

Additionally, the document contains examples of how these extensions
should be used and why. For example, using alias templates over class
templates.

llvm-svn: 363110
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 4959542657ba7965db0f34f630830000bbdd6563
diff --git a/docs/index.rst b/docs/index.rst
index 37b278f..e2aca72 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -142,6 +142,7 @@
    DesignDocs/ThreadingSupportAPI
    DesignDocs/FileTimeType
    DesignDocs/FeatureTestMacros
+   DesignDocs/ExtendedCXX03Support
 
 * `<atomic> design <http://libcxx.llvm.org/atomic_design.html>`_
 * `<type_traits> design <http://libcxx.llvm.org/type_traits_design.html>`_