[libcxx] Remove unexpected handlers in C++17

Summary:
This patch implements [P0003R5](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0003r5.html) which removes exception specifications from C++17.

The only changes to the library are removing `set_unexpected`, `get_unexpected`, `unexpected`, and `unexpected_handler`. These functions can be re-enabled in C++17 using `_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS`.

@mclow.lists what do you think about removing stuff is this way?

Reviewers: mclow.lists

Reviewed By: mclow.lists

Subscribers: mclow.lists, cfe-commits

Differential Revision: https://reviews.llvm.org/D28172

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@295406 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/depr/exception.unexpected/unexpected.pass.cpp

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// test unexpected
+
+// MODULES_DEFINES: _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS
+#define _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS
+#include <exception>
+#include <cstdlib>
+#include <cassert>
+
+void fexit()
+{
+    std::exit(0);
+}
+
+int main()
+{
+    std::set_unexpected(fexit);
+    std::unexpected();
+    assert(false);
+}