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/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.fail.cpp

@@ -1,33 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// UNSUPPORTED: c++98, c++03
-
-// <tuple>
-
-// template <class... Types> class tuple;
-
-// template <class Alloc>
-//   explicit(see-below) tuple(allocator_arg_t, const Alloc& a);
-
-// Make sure we get the explicit-ness of the constructor right.
-// This is LWG 3158.
-
-#include <tuple>
-#include <memory>
-
-
-struct ExplicitDefault { explicit ExplicitDefault() { } };
-
-std::tuple<ExplicitDefault> explicit_default_test() {
-    return {std::allocator_arg, std::allocator<int>()}; // expected-error {{chosen constructor is explicit in copy-initialization}}
-}
-
-int main(int, char**) {
-    return 0;
-}

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp

@@ -13,7 +13,7 @@
 // template <class... Types> class tuple;
 
 // template <class Alloc>
-//   explicit(see-below) tuple(allocator_arg_t, const Alloc& a);
+//   tuple(allocator_arg_t, const Alloc& a);
 
 // NOTE: this constructor does not currently support tags derived from
 // allocator_arg_t because libc++ has to deduce the parameter as a template

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.fail.cpp

@@ -1,53 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// UNSUPPORTED: c++98, c++03
-
-// Before GCC 6, aggregate initialization kicks in.
-// See https://stackoverflow.com/q/41799015/627587.
-// UNSUPPORTED: gcc-5
-
-// <tuple>
-
-// template <class... Types> class tuple;
-
-// explicit(see-below) constexpr tuple();
-
-#include <tuple>
-
-
-struct Implicit {
-    Implicit() = default;
-};
-
-struct Explicit {
-    explicit Explicit() = default;
-};
-
-std::tuple<> test1() { return {}; }
-
-std::tuple<Implicit> test2() { return {}; }
-std::tuple<Explicit> test3() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-
-std::tuple<Implicit, Implicit> test4() { return {}; }
-std::tuple<Explicit, Implicit> test5() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-std::tuple<Implicit, Explicit> test6() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-std::tuple<Explicit, Explicit> test7() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-
-std::tuple<Implicit, Implicit, Implicit> test8() { return {}; }
-std::tuple<Implicit, Implicit, Explicit> test9() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-std::tuple<Implicit, Explicit, Implicit> test10() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-std::tuple<Implicit, Explicit, Explicit> test11() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-std::tuple<Explicit, Implicit, Implicit> test12() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-std::tuple<Explicit, Implicit, Explicit> test13() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-std::tuple<Explicit, Explicit, Implicit> test14() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-std::tuple<Explicit, Explicit, Explicit> test15() { return {}; } // expected-error 1 {{chosen constructor is explicit in copy-initialization}}
-
-int main(int, char**) {
-    return 0;
-}

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/default.pass.cpp

@@ -10,7 +10,7 @@
 
 // template <class... Types> class tuple;
 
-// explicit(see-below) constexpr tuple();
+// constexpr tuple();
 
 // UNSUPPORTED: c++98, c++03
 

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_incomplete.pass.cpp

@@ -14,6 +14,7 @@
 //   struct tuple_size<tuple<Types...>>
 //     : public integral_constant<size_t, sizeof...(Types)> { };
 
+// XFAIL: gcc-4.9
 // UNSUPPORTED: c++98, c++03
 
 #include <tuple>

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_value_sfinae.pass.cpp

@@ -14,6 +14,7 @@
 //   class tuple_size<tuple<Types...>>
 //     : public integral_constant<size_t, sizeof...(Types)> { };
 
+// XFAIL: gcc-4.9
 // UNSUPPORTED: c++98, c++03
 
 #include <tuple>