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

[Stephan T. Lavavej 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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StephanTLavavej wrote:

>From a user perspective, having compilers align on a particular value in the range "greater than C++23's value, but less than C++26's final value" doesn't seem important. But from Clang's perspective, I can see how it would be simpler to avoid having to vary depending on whether you're imitating MSVC or not. I'll make this change for our implementation.

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