Add std::is_constant_evaluated.

Clang recently added __builtin_is_constant_evaluated() and GCC 9.0
has it as well.

This patch adds support for it in libc++.

llvm-svn: 359119
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
index c11846e..ed01587 100755
--- a/libcxx/utils/generate_feature_test_macro_components.py
+++ b/libcxx/utils/generate_feature_test_macro_components.py
@@ -525,7 +525,8 @@
      "c++2a": 201811L,
    },
    "headers": ["type_traits"],
-   "unimplemented": True,
+   "depends": "TEST_HAS_BUILTIN(__builtin_is_constant_evaluated) || TEST_GCC_VER >= 900",
+   "internal_depends": "!defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)",
    },
   {"name": "__cpp_lib_list_remove_return_type",
    "values": {