Revert r372777: [libc++] Implement LWG 2510 and its follow-ups

This also reverts:
 - r372778: [libc++] Implement LWG 3158
 - r372782: [libc++] Try fixing tests that fail on GCC 5 and older
 - r372787: Purge mentions of GCC 4 from the test suite

Reason: the change breaks compilation of LLVM with libc++, for details see
http://lists.llvm.org/pipermail/libcxx-dev/2019-September/000599.html

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@372832 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/language.support/support.dynamic/nothrow_t.pass.cpp

@@ -1,23 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-//                     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.
-//
-//===----------------------------------------------------------------------===//
-
-// struct nothrow_t {
-//   explicit nothrow_t() = default;
-// };
-// extern const nothrow_t nothrow;
-
-#include <new>
-
-
-int main(int, char**) {
-  std::nothrow_t x = std::nothrow;
-  (void)x;
-
-  return 0;
-}