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

llvm-svn: 372832
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: a3d337a9a7d00d82bb190c5e551181d3998f6b98
diff --git a/include/new b/include/new
index 40d351e..85e4c4b 100644
--- a/include/new
+++ b/include/new
@@ -39,7 +39,7 @@
 };
 inline constexpr destroying_delete_t destroying_delete{}; // C++20
 
-struct nothrow_t { explicit nothrow_t() = default; };
+struct nothrow_t {};
 extern const nothrow_t nothrow;
 typedef void (*new_handler)();
 new_handler set_new_handler(new_handler new_p) noexcept;
@@ -126,7 +126,7 @@
 {
 
 #if !defined(_LIBCPP_ABI_VCRUNTIME)
-struct _LIBCPP_TYPE_VIS nothrow_t { explicit nothrow_t() = default; };
+struct _LIBCPP_TYPE_VIS nothrow_t {};
 extern _LIBCPP_FUNC_VIS const nothrow_t nothrow;
 
 class _LIBCPP_EXCEPTION_ABI bad_alloc