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

[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,
----------------
bgra8 wrote:

> we have an existing test that embeds a null character into the source and that doesn't seem to reproduce the issue

Can you construct a test case with an embedded null character into the source and make the call to `PP->getCheckPoint(FID, FirstLineStart))` (at `clang/lib/Frontend/TextDiagnostic.cpp:1155` also shown below) return an address _after_ the null? That would reproduce the issue.

```
  const char *FirstLineStart =
      FileData.data() +
      SM.getDecomposedLoc(SM.translateLineCol(FID, StartLineNumber, 1)).second;
  if (const char *CheckPoint = PP->getCheckPoint(FID, FirstLineStart)) {
    assert(CheckPoint >= Buff->getBufferStart() &&
           CheckPoint <= Buff->getBufferEnd());
    assert(CheckPoint <= FirstLineStart);
    size_t Offset = CheckPoint - Buff->getBufferStart();
    L.seek(Offset, /*IsAtStartOfLine=*/false);
  }
```

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