[C++1z] Add speculative SD-6 feature test macro for pack expansions in using declarations

[EricWF]

Summary

This patch adds the __cpp_variadic_using feature test macro suggested in P0195r2. Note that this macro is not yet listed by SD-6.

lib/Frontend/InitPreprocessor.cpp

if (LangOpts.CPlusPlus1z) {
     Builder.defineMacro("__cpp_nested_namespace_definitions", "201411");
     Builder.defineMacro("__cpp_variadic_using", "201611");
     Builder.defineMacro("__cpp_aggregate_bases", "201603");
     Builder.defineMacro("__cpp_structured_bindings", "201606");
     Builder.defineMacro("__cpp_nontype_template_args", "201411");
     Builder.defineMacro("__cpp_fold_expressions", "201603");
     // FIXME: This is not yet listed in SD-6.
     Builder.defineMacro("__cpp_deduction_guides", "201611");
+    Builder.defineMacro("__cpp_variadic_using", "201611");
   }
   if (LangOpts.AlignedAllocation)
     Builder.defineMacro("__cpp_aligned_new", "201606");
 
   // TS features.
   if (LangOpts.ConceptsTS)
     Builder.defineMacro("__cpp_experimental_concepts", "1");
   if (LangOpts.CoroutinesTS)

test/Lexer/cxx-features.cpp

 #if check(template_template_args, 0, 0, 0, 0) // FIXME: should be 201611 when feature is enabled
 #error "wrong value for __cpp_template_template_args"
 #endif
 
 #if check(deduction_guides, 0, 0, 0, 201611) // FIXME: provisional name
 #error "wrong value for __cpp_deduction_guides"
 #endif
 
+#if check(variadic_using, 0, 0, 0, 201611) // FIXME: provisional name
+#error "wrong value for __cpp_variadic_using"
+#endif
+
 // --- C++14 features ---
 
 #if check(binary_literals, 0, 0, 201304, 201304)
 #error "wrong value for __cpp_binary_literals"
 #endif
 
 // (Removed from SD-6.)
 #if check(digit_separators, 0, 0, 201309, 201309)