[coroutines] Bump __cpp_coroutines version

Summary: This patch is needed so that Libc++ can actually tess if Clang supports coroutines, instead of just paying lip service with a partial implementation. Otherwise the libc++ test suite will fail against older versions of Clang

Reviewers: GorNishanov, rsmith

Reviewed By: GorNishanov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33536

llvm-svn: 303867
diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 9257dca..08befb3 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -535,7 +535,7 @@
   if (LangOpts.ConceptsTS)
     Builder.defineMacro("__cpp_experimental_concepts", "1");
   if (LangOpts.CoroutinesTS)
-    Builder.defineMacro("__cpp_coroutines", "1");
+    Builder.defineMacro("__cpp_coroutines", "201703L");
 }
 
 static void InitializePredefinedMacros(const TargetInfo &TI,