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.fail.cpp

@@ -1,30 +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.
-//
-//===----------------------------------------------------------------------===//
-
-// UNSUPPORTED: c++98, c++03
-
-// Before GCC 6, aggregate initialization kicks in.
-// See https://stackoverflow.com/q/41799015/627587.
-// UNSUPPORTED: gcc-5
-
-// struct nothrow_t {
-//   explicit nothrow_t() = default;
-// };
-// extern const nothrow_t nothrow;
-
-// This test checks for LWG 2510.
-
-#include <new>
-
-
-std::nothrow_t f() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-
-int main(int, char**) {
-    return 0;
-}