[libcxx] Only enable the availability LIT feature when we're testing libc++

Other standard libraries don't implement availability markup, so it doesn't
make sense to e.g. XFAIL tests based on availability markup outside of
libc++.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348871 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/libcxx/test/config.py b/utils/libcxx/test/config.py
index c944b6b..1aa52dd 100644
--- a/utils/libcxx/test/config.py
+++ b/utils/libcxx/test/config.py
@@ -407,8 +407,10 @@
             if self.use_deployment:
                 self.add_deployment_feature('with_system_cxx_lib')
 
-        # Configure the availability feature.
-        if self.use_deployment:
+        # Configure the availability feature. Availability is only enabled
+        # with libc++, because other standard libraries do not provide
+        # availability markup.
+        if self.use_deployment and self.cxx_stdlib_under_test == 'libc++':
             self.config.available_features.add('availability')
             self.add_deployment_feature('availability')