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

[Corentin Jabot via cfe-commits]

================
@@ -103,8 +103,10 @@ SourceRange Preprocessor::DiscardUntilEndOfDirective(
 
 /// Enumerates possible cases of #define/#undef a reserved identifier.
 enum MacroDiag {
-  MD_NoWarn,        //> Not a reserved identifier
-  MD_KeywordDef,    //> Macro hides keyword, enabled by default
+  MD_NoWarn,       //> Not a reserved identifier
+  MD_KeywordDef,   //> Macro hides keyword, enabled by default
+  MD_KeywordUnDef, //> Undef keyword,  It is generally harmless and widely used,
----------------
cor3ntin wrote:

```suggestion
  MD_KeywordUndef, //> Undef keyword,  It is generally harmless and widely used,
```

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