[libc++] Raise an exception if a Lit feature resolves to an invalid name

This allows reporting issues early when creating feature names.

libcxx/test/libcxx/selftest/dsl/dsl.sh.py

@@ -194,6 +194,16 @@ class TestFeature(SetupConfigs):
         feature.enableIn(self.config)
         self.assertIn('name', self.config.available_features)
 
+    def test_name_is_not_a_string_1(self):
+        feature = dsl.Feature(name=None)
+        assert feature.isSupported(self.config)
+        self.assertRaises(ValueError, lambda: feature.enableIn(self.config))
+
+    def test_name_is_not_a_string_2(self):
+        feature = dsl.Feature(name=lambda cfg: None)
+        assert feature.isSupported(self.config)
+        self.assertRaises(ValueError, lambda: feature.enableIn(self.config))
+
     def test_adding_compile_flag(self):
         feature = dsl.Feature(name='name', compileFlag='-foo')
         origLinkFlags = copy.deepcopy(self.getSubstitution('%{link_flags}'))

libcxx/utils/libcxx/test/dsl.py

@@ -220,6 +220,8 @@ class Feature(object):
       config.substitutions = addTo(config.substitutions, '%{link_flags}', linkFlag)
 
     name = self._name(config) if callable(self._name) else self._name
+    if not isinstance(name, str):
+      raise ValueError("Feature did not resolve to a name that's a string, got {}".format(name))
     config.available_features.add(name)