[coroutines] Bump __cpp_coroutines version

[EricWF]

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

[GorNishanov]

LGTM

lib/Frontend/InitPreprocessor.cpp

static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   }
   if (LangOpts.AlignedAllocation)
     Builder.defineMacro("__cpp_aligned_new", "201606");
 
   // TS features.
   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,
                                        const LangOptions &LangOpts,
                                        const FrontendOptions &FEOpts,
                                        MacroBuilder &Builder) {
   // Compiler version introspection macros.
   Builder.defineMacro("__llvm__");  // LLVM Backend

test/Lexer/cxx-features.cpp

 #endif
 
 // --- TS features --
 
 #if check(experimental_concepts, 0, 0, CONCEPTS_TS, CONCEPTS_TS)
 #error "wrong value for __cpp_experimental_concepts"
 #endif
 
-#if (COROUTINES && !__cpp_coroutines) || (!COROUTINES && __cpp_coroutines)
+#if defined(COROUTINES) ? check(coroutines, 201703L, 201703L, 201703L, 201703L) : check(coroutines, 0, 0, 0, 0)
 #error "wrong value for __cpp_coroutines"
 #endif