[libcxx] Slightly improved policy for handling experimental features

Summary:
Following the discussion on the libcxx-dev mailing list
(http://lists.llvm.org/pipermail/libcxx-dev/2019-May/000358.html),
this implements the new policy for handling experimental features and
their deprecation. We basically add a deprecation warning for
std::experimental::filesystem, and we remove a bunch of <experimental/*>
headers that were now empty.

Reviewers: mclow.lists, EricWF

Subscribers: mgorny, christof, jkorous, dexonsmith, arphaman, libcxx-commits, jfb

Tags: #libc

Differential Revision: https://reviews.llvm.org/D62428

llvm-svn: 363072
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 776acf225b371b4f4623751a3de8c2d3d324473c
diff --git a/docs/UsingLibcxx.rst b/docs/UsingLibcxx.rst
index ef3023e..56c98af 100644
--- a/docs/UsingLibcxx.rst
+++ b/docs/UsingLibcxx.rst
@@ -84,6 +84,9 @@
     * The contents of the ``<experimental/...>`` headers and ``libc++experimental.a``
       library will not remain compatible between versions.
     * No guarantees of API or ABI stability are provided.
+    * When we implement the standardized version of an experimental feature,
+      the experimental feature is removed two releases after the non-experimental
+      version has shipped. The full policy is explained :ref:`here <experimental features>`.
 
 Using libc++ on Linux
 =====================