[coroutines] Add feature-test macro for coroutines, defined to 1 to indicate

the implementation is incomplete.

llvm-svn: 250982

clang/lib/Frontend/InitPreprocessor.cpp

@@ -456,6 +456,8 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_sized_deallocation", "201309");
   if (LangOpts.ConceptsTS)
     Builder.defineMacro("__cpp_experimental_concepts", "1");
+  if (LangOpts.Coroutines)
+    Builder.defineMacro("__cpp_coroutines", "1");
 }
 
 static void InitializePredefinedMacros(const TargetInfo &TI,