[C++20] [Modules] Set the feature testing macro to 1 (#161034)

See #71364 for details.

Author: ChuanqiXu9

clang/lib/Frontend/InitPreprocessor.cpp

@@ -742,7 +742,10 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_impl_coroutine", "201902L");
     Builder.defineMacro("__cpp_designated_initializers", "201707L");
     Builder.defineMacro("__cpp_impl_three_way_comparison", "201907L");
-    //Builder.defineMacro("__cpp_modules", "201907L");
+    // Intentionally to set __cpp_modules to 1.
+    // See https://github.com/llvm/llvm-project/issues/71364 for details.
+    // Builder.defineMacro("__cpp_modules", "201907L");
+    Builder.defineMacro("__cpp_modules", "1");
     Builder.defineMacro("__cpp_using_enum", "201907L");
   }
   // C++23 features.

clang/test/Lexer/cxx-features.cpp

@@ -148,7 +148,7 @@
 
 // init_captures checked below
 
-#if check(modules, 0, 0, 0, 0, 0, 0, 0)
+#if check(modules, 0, 0, 0, 0, 1, 1, 1)
 // FIXME: 201907 in C++20
 #error "wrong value for __cpp_modules"
 #endif