[clang] Fix clang crash when printing highlighted code in diagnostic (after #66514) (PR #80442)

[Aaron Ballman via cfe-commits]

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@@ -1349,7 +1349,7 @@ void TextDiagnostic::emitSnippetAndCaret(
   // Prepare source highlighting information for the lines we're about to
   // emit, starting from the first line.
   std::unique_ptr<SmallVector<StyleRange>[]> SourceStyles =
-      highlightLines(BufStart, Lines.first, Lines.second, PP, LangOpts,
+      highlightLines(BufData, Lines.first, Lines.second, PP, LangOpts,
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AaronBallman wrote:

Thank you for the debugging help! You're right, the scratch buffer does embed null characters: https://github.com/llvm/llvm-project/blob/ae354c5a45d319b3117c2822b8f6988461f3cb33/clang/lib/Lex/ScratchBuffer.cpp#L62

That gets used for the stringify operator (`#`) as well as for the `__DATE__` and `__TIME__` predefined macros, and the `_Pragma` operator; hopefully that helps @tbaederr to find a way to add test coverage for this so we don't accidentally regress it in the future.

https://github.com/llvm/llvm-project/pull/80442