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

[Shafik Yaghmour via cfe-commits]

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@@ -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.
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shafik wrote:

I am a bit uncomfortable just putting a link w/o at least a TL;DR; giving a high level explanation here. 

https://github.com/llvm/llvm-project/pull/161034