[libcxx] Add new Sphinx documentation

Summary:
This patch adds Sphinx based documentation to libc++. The goal is to make it easier to write documentation for libc++ since writing new documentation in HTML is cumbersome. This patch rewrites the main page for libc++ along with the instructions for using, building and testing libc++. 

The built documentation can be found and reviewed here: http://efcs.ca/libcxx-docs

In order to build the sphinx documentation you need to specify the cmake options `-DLLVM_ENABLE_SPHINX=ON -DLIBCXX_INCLUDE_DOCS=ON`. This will add the makefile rule `docs-libcxx-html`.

Reviewers: chandlerc, mclow.lists, danalbert, jroelofs

Subscribers: silvas, cfe-commits

Differential Revision: http://reviews.llvm.org/D12129

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@245788 91177308-0d34-0410-b5e6-96231b3b80d8

docs/UsingLibcxx.rst

@@ -0,0 +1,89 @@
+============
+Using libc++
+============
+
+.. contents::
+  :local:
+
+Getting Started
+===============
+
+If you already have libc++ installed you can use it with clang.
+
+.. code-block:: bash
+
+    $ clang++ -stdlib=libc++ test.cpp
+    $ clang++ -std=c++11 -stdlib=libc++ test.cpp
+
+On OS X and FreeBSD libc++ is the default standard library
+and the ``-stdlib=libc++`` is not required.
+
+.. _alternate libcxx:
+
+If you want to select an alternate installation of libc++ you
+can use the following options.
+
+.. code-block:: bash
+
+  $ clang++ -std=c++11 -stdlib=libc++ -nostdinc++ \
+            -I<libcxx-install-prefix>/include/c++/v1 \
+            -L<libcxx-install-prefix>/lib \
+            -Wl,-rpath,<libcxx-install-prefix>/lib \
+            test.cpp
+
+The option ``-Wl,-rpath,<libcxx-install-prefix>/lib`` adds a runtime library
+search path. Meaning that the systems dynamic linker will look for libc++ in
+``<libcxx-install-prefix>/lib`` whenever the program is run. Alternatively the
+environment variable ``LD_LIBRARY_PATH`` (``DYLD_LIBRARY_PATH`` on OS X) can
+be used to change the dynamic linkers search paths after a program is compiled.
+
+An example of using ``LD_LIBRARY_PATH``:
+
+.. code-block:: bash
+
+  $ clang++ -stdlib=libc++ -nostdinc++ \
+            -I<libcxx-install-prefix>/include/c++/v1
+            -L<libcxx-install-prefix>/lib \
+            test.cpp -o
+  $ ./a.out # Searches for libc++ in the systems library paths.
+  $ export LD_LIBRARY_PATH=<libcxx-install-prefix>/lib
+  $ ./a.out # Searches for libc++ along LD_LIBRARY_PATH
+
+
+
+Using libc++ on Linux
+=====================
+
+On Linux libc++ typically links to a shared version of libc++abi. Unfortunately
+you can't simply run clang with "-stdlib=libc++" as clang is not set up to
+link for this configuration. To get around this you'll have to manually
+link libc++abi yourself. For example:
+
+.. code-block:: bash
+
+  $ clang++ -stdlib=libc++ test.cpp -lc++ -lc++abi -lm -lc -lgcc_s -lgcc
+
+Alternately, you could just add libc++abi to your libraries list, which in
+most situations will give the same result:
+
+.. code-block:: bash
+
+  $ clang++ -stdlib=libc++ test.cpp -lc++abi
+
+
+Using libc++ with GCC
+---------------------
+
+GCC does not provide a way to switch from libstdc++ to libc++. You must manually
+configure the compile and link commands.
+
+In particular you must tell GCC to remove the libstdc++ include directories
+using ``-nostdinc++`` and to not link libstdc++.so using ``-nodefaultlibs``.
+
+Note that ``-nodefaultlibs`` removes all of the standard system libraries and
+not just libstdc++ so they must be manually linked. For example:
+
+.. code-block:: bash
+
+  $ g++ -nostdinc++ -I<libcxx-install-prefix>/include/c++/v1 \
+         test.cpp -nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc_s -lgcc