Fix most GCC test failures.

This patch fixes almost all currently failing tests when
using GCC ToT.

The specific changes are:

(A) Workaround gcc.gnu.org/PR83921 which rejects variables w/o initializers
in constexpr contexts -- even when the variable is an empty class. This
bug has been worked around at all callsites by adding an initializer.
Additionally a new test, constexpr_init.pass.cpp, has been added to
test that Clang doesn't suffer from these bugs.

(B) Fix streambuf.assign/swap.pass.cpp. This test was never actually
calling the swap method as intended. In fact, the swap function it
intended to call was ill-formed when instantiated. GCC diagnosed
this ill-formedness w/o needing an instantiation.

(C) size_delete11.pass.cpp was fixed by adding c++2a to the list of
unsupported dialects.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@322810 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/comparisons/constexpr_init.pass.cpp

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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, c++11
+
+// XFAIL: gcc-7, gcc-8
+
+// <functional>
+
+// equal_to, not_equal_to, less, et al.
+
+// Test that these types can be constructed w/o an initializer in a constexpr
+// context. This is specifically testing gcc.gnu.org/PR83921
+
+
+#include <functional>
+#include "test_macros.h"
+
+template <class T>
+constexpr bool test_constexpr_context() {
+  std::equal_to<T> eq;
+  ((void)eq);
+  std::not_equal_to<T> neq;
+  ((void)neq);
+  std::less<T> l;
+  ((void)l);
+  std::less_equal<T> le;
+  ((void)le);
+  std::greater<T> g;
+  ((void)g);
+  std::greater_equal<T> ge;
+  ((void)ge);
+  return true;
+}
+
+static_assert(test_constexpr_context<int>(), "");
+static_assert(test_constexpr_context<void>(), "");
+
+
+int main() {
+
+}