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/iterators/stream.iterators/istream.iterator/types.pass.cpp

@@ -8,6 +8,9 @@
 
 // <iterator>
 
+// Test fails due to use of is_trivially_* trait.
+// XFAIL: gcc-4.9
+
 // template <class T, class charT = char, class traits = char_traits<charT>,
 //           class Distance = ptrdiff_t>
 // class istream_iterator

test/std/iterators/stream.iterators/istreambuf.iterator/types.pass.cpp

@@ -8,6 +8,9 @@
 
 // <iterator>
 
+// Test fails due to use of is_trivially_* trait.
+// XFAIL: gcc-4.9
+
 // template<class charT, class traits = char_traits<charT> >
 // class istreambuf_iterator
 //     : public iterator<input_iterator_tag, charT,