Mark a couple of tests as XFAIL with older compilers.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223600 91177308-0d34-0410-b5e6-96231b3b80d8

test/atomics/atomics.types.generic/trivially_copyable.pass.cpp

@@ -9,6 +9,10 @@
 //
 // UNSUPPORTED: libcpp-has-no-threads
 
+// NOTE: atomic<> of a TriviallyCopyable class is wrongly rejected by older
+// clang versions. It was fixed right before the llvm 3.5 release. See PR18097.
+// XFAIL: apple-clang-6.0, clang-3.4, clang-3.3
+
 // <atomic>
 
 // template <class T>

test/experimental/string.view/string.view.access/at.pass.cpp

@@ -7,6 +7,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// NOTE: Older versions of clang have a bug where they fail to evalute
+// string_view::at as a constant expression.
+// XFAIL: apple-clang-6.0, clang-3.4, clang-3.3
 
 // <string_view>