[clang] [Clang] Add flags and lang option for C++2d (PR #203992)

[A. Jiang via cfe-commits]

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@@ -228,7 +228,10 @@ static void addVisualCDefines(const LangOptions &Opts, MacroBuilder &Builder) {
       Builder.defineMacro("_HAS_CHAR16_T_LANGUAGE_SUPPORT", Twine(1));
 
     if (Opts.isCompatibleWithMSVC(LangOptions::MSVC2015)) {
-      if (Opts.CPlusPlus26)
+      if (Opts.CPlusPlus29)
+        // TODO update to the proper value.
+        Builder.defineMacro("_MSVC_LANG", "202700L");
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frederick-vs-ja wrote:

BTW, for Clang-cl, it seems that we should also have `-std:c++26preview` option and map `-std:c++latest` to `-std=c++2d`. But we should to discuss this with MSVC team first.

https://github.com/llvm/llvm-project/pull/203992