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.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>