[libc++] Fix failing concepts tests

libcxx/test/std/concepts/lang/same_as.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
 
 // template<class T, class U>
 // concept same_as;

libcxx/utils/libcxx/test/config.py

@@ -472,6 +472,10 @@ class Configuration(object):
                 intMacroValue(macros['__cpp_deduction_guides']) < 201611:
             self.config.available_features.add('libcpp-no-deduction-guides')
 
+        if '__cpp_concepts' not in macros or \
+                intMacroValue(macros['__cpp_concepts']) < 201811:
+            self.config.available_features.add('libcpp-no-concepts')
+
         if self.target_info.is_windows():
             self.config.available_features.add('windows')
             if self.cxx_stdlib_under_test == 'libc++':