[clang-format] Skip parsing a block if it's nested too deep

[owenpan]

Summary

Don't parse blocks that are nested thousands of levels deep, e.g., https://github.com/llvm/llvm-project/blob/a7b154aa1770a2223be2e99735fab54c1c081007/clang/test/Parser/parser_overflow.c#L11.

Adds an upper bound to the number of levels of nested blocks to provide a safeguard.

Fixes https://github.com/llvm/llvm-project/issues/55912.

[owenpan]

Rebased.

[owenpan]

Inadvertently moved the added code down when rebasing. Fixed.

clang/lib/Format/UnwrappedLineParser.cpp

void UnwrappedLineParser::parseBlock(bool MustBeDeclaration, unsigned AddLevels,
   if (AddLevels > 0 && Style.BreakBeforeBraces == FormatStyle::BS_Whitesmiths)
     ++Line->Level;
 
   size_t PPStartHash = computePPHash();
 
   unsigned InitialLevel = Line->Level;
   nextToken(/*LevelDifference=*/AddLevels);
 
+  // Bail out if there are too many levels. Otherwise, the stack might overflow.
+  if (Line->Level > 300)
+    return;
+
   if (MacroBlock && FormatTok->is(tok::l_paren))
     parseParens();
 
   size_t NbPreprocessorDirectives =
       CurrentLines == &Lines ? PreprocessorDirectives.size() : 0;
   addUnwrappedLine();
   size_t OpeningLineIndex =
       CurrentLines->empty()