[clang] [clang] Improve diagnostics for invalid named-universal-characters (PR #206326)

[Corentin Jabot via cfe-commits]

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@@ -539,6 +539,10 @@ static bool ProcessNumericUCNEscape(const char *ThisTokBegin,
   return !HasError;
 }
 
+static bool AllowedInCharacterName(char C) {
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cor3ntin wrote:

We are very inconsistent with that, Timm

https://github.com/llvm/llvm-project/pull/206326