[clang] clang: Relax LangOpts checks when lexing quoted numbers during preprocessing (PR #95798)

[Jan Svoboda via cfe-commits]

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@@ -2068,7 +2068,8 @@ bool Lexer::LexNumericConstant(Token &Result, const char *CurPtr) {
   }
 
   // If we have a digit separator, continue.
-  if (C == '\'' && (LangOpts.CPlusPlus14 || LangOpts.C23)) {
+  if (C == '\'' &&
+      (LangOpts.CPlusPlus14 || LangOpts.C23 || ParsingPreprocessorDirective)) {
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jansvoboda11 wrote:

We've been trying to avoid specializing Clang itself for dependency scanning. Instead, we have introduced generic feature flags that the scanner uses to tweak the compiler behavior. I think this is better for layering and also forces us to name those features, making the code self-documenting. People working on Clang don't have to wonder what's dependency scanning and why it needs to lex numeric literals differently; they just see there's a feature flag named `AllowLiteralDigitSeparator` that just makes sense in that context. You can still dive deeper and find that it's the scanner that enables that feature of course.

So I'm not a fan of introducing `LanguageOpts::ScanningDependencies`.

https://github.com/llvm/llvm-project/pull/95798