Have `__have_extension(cxx_variadic_templates)` return true for any C++ standard.

[EricWF]

Summary

Currently __has_extension(cxx_variadic_templates) always evaluates to 0 even though the front-end accepts the extension.

[EricWF]

Gentle ping. This is a tiny change but I don't work on clang so I would really like a thumbs up.

[rsmith]

LGTM

[rsmith]

lib/Lex/PPMacroExpansion.cpp

1239

Please keep this list alphabetized.

[EricWF]

Alphabetize entry in list.

[EricWF]

Closed in r237202.

lib/Lex/PPMacroExpansion.cpp

            .Case("cxx_range_for", LangOpts.CPlusPlus)
            .Case("cxx_reference_qualified_functions", LangOpts.CPlusPlus)
            .Case("cxx_rvalue_references", LangOpts.CPlusPlus)
+           .Case("cxx_variadic_templates", LangOpts.CPlusPlus)
            // C++1y features supported by other languages as extensions.
            .Case("cxx_binary_literals", true)
            .Case("cxx_init_captures", LangOpts.CPlusPlus11)

test/Lexer/has_extension_cxx.cpp

 int has_rvalue_references();
 #endif
 
+// CHECK: has_variadic_templates
+#if __has_extension(cxx_variadic_templates)
+int has_variadic_templates();
+#endif
+
 // CHECK: has_local_type_template_args
 #if __has_extension(cxx_local_type_template_args)
 int has_local_type_template_args();