Mark up clang failure

[SVN r61894]

status/explicit-failures-markup.xml

@@ -5778,7 +5778,20 @@ These tests will fail in most compilers that don't support rvalue references.
         <mark-expected-failures>
             <test name="fail_base_dimension"/>
             <toolset name="vacpp"/>
-            <note author="Steven Watanabe" refid="16"/>
+            <note author="Steven Watanabe">
+                The library depends on undefined behavior to generate
+                better errors.  This failure should not affect most
+                users of the library.
+            </note>
+        </mark-expected-failures>
+        <mark-expected-failures>
+            <test name="fail_base_dimension"/>
+            <toolset name="clang-darwin"/>
+            <note author="Steven Watanabe">
+                The library depends on undefined behavior to generate
+                better errors.  This failure should not affect most
+                users of the library.
+            </note>
         </mark-expected-failures>
     </library>