[libc++] Mark a few tests as unsupported on older Clangs to fix bots

Author: ldionne

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate_hint.pass.cpp

@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: clang-8
+
 // <memory>
 
 // template <class Alloc>

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/construct.pass.cpp

@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: clang-8
+
 // <memory>
 
 // template <class Alloc>

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/destroy.pass.cpp

@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: clang-8
+
 // <memory>
 
 // template <class Alloc>

libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp

@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: clang-8
+
 // <memory>
 
 // allocator: