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

[Erich Keane 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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erichkeane wrote:

So this should only matter if there is a null-terminator in the `BufData`, right?  I'm really curious what could have caused that.  It would be appreciated if we could still get a reduced version of the test for a lit-test, but as this is blocking your downstream, I can accept without a repro.

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