[clang] [C++26] Implement P2843R3 - Preprocessing is never undefined (define/undef keyword part) (PR #218322)

[Yihan Wang via cfe-commits]

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@@ -412,6 +423,8 @@ bool Preprocessor::CheckMacroName(Token &MacroNameTok, MacroUse isDefineUndef,
       if (ShadowFlag)
         *ShadowFlag = true;
     }
+    if (D == MD_KeywordUndef)
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yronglin wrote:

Fixed

https://github.com/llvm/llvm-project/pull/218322