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

Implements the fix proposed by Evgeny Eltsin on
#66514 (comment).

No test case provided, since the bug is extremely sensitive to the
preprocessor
state (headers, macros, including the ones defined on command line), and
it
turned out to be non-trivial to create an isolated test.

Author: alexfh

clang/lib/Frontend/TextDiagnostic.cpp

@@ -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,
                      DiagOpts->ShowColors, FID, SM);
 
   SmallVector<LineRange> LineRanges =