[libcxx] Slightly improved policy for handling experimental features

Following the discussion on the libcxx-dev mailing list
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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@363072 91177308-0d34-0410-b5e6-96231b3b80d8
36 files changed