[clang] [clang-cl]: Add /std:c++23preview and update _MSVC_LANG for C++23 (PR #112378)

[Aaron Ballman via cfe-commits]

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@@ -215,8 +215,7 @@ static void addVisualCDefines(const LangOptions &Opts, MacroBuilder &Builder) {
 
     if (Opts.isCompatibleWithMSVC(LangOptions::MSVC2015)) {
       if (Opts.CPlusPlus23)
-        // TODO update to the proper value.
-        Builder.defineMacro("_MSVC_LANG", "202004L");
+        Builder.defineMacro("_MSVC_LANG", "202302L");
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AaronBallman wrote:

Clang and GCC both use `202400L`: https://godbolt.org/z/7er4doE6h -- maybe Microsoft wants to consider using the same value?

https://github.com/llvm/llvm-project/pull/112378