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

[Aaron Ballman 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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AaronBallman wrote:

There's not a one-to-one mapping of `LangOptions` to frontend of driver options, yes. But I think it's a stretch to argue that the standard mode tracked by `LangOptions` is not exposed via flags.

I don't think per-feature `LangOptions` is viable. There's digit separators, hex float support, binary literal support, different literal suffixes, various keywords, etc and that doesn't seem likely to scale well. Also, it makes it seem like we support a la carte language features when we don't -- we intentionally don't want to let users disable standard features (in general; exceptions exist) and so that approach gives the impression contributors need to check both language mode and feature availability which is a maintenance concern.

I'm still wondering about the question I asked here: https://github.com/llvm/llvm-project/pull/93753#issuecomment-2173666602

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