[clang-format] Refactoring and asserts in LevelIndentTracker. (NFC)

[Sedeniono]

Summary

• adjustToUnmodifiedLine: The code does something only for non-PP-directives. This is now reflected by putting the if-check to the top. This also ensures that the assert() there is executed only if IndentForLevel is actually accessed.

• getIndent(): assert valid index into IndentForLevel.

• Added explanation regarding the intention of IndentForLevel.

This refactoring was split from https://reviews.llvm.org/D151047.
But both patches are independent of each other.

[owenpan]

clang/lib/Format/UnwrappedLineFormatter.cpp

91

Early return.

[owenpan]

clang/lib/Format/UnwrappedLineFormatter.cpp

92–96

Ditto.

[Sedeniono]

Implemented review changes suggested by @owenpan.

[Sedeniono]

I don't see how the failed test cases could be my fault; they deal with the address sanitizer.
So maybe some one could please commit this to main using Sedenion <[e-mail]>? Thanks!

clang/lib/Format/UnwrappedLineFormatter.cpp

public:
   }
 
   /// Update the level indent to adapt to the given \p Line.
   ///
   /// When a line is not formatted, we move the subsequent lines on the same
   /// level to the same indent.
   /// Note that \c nextLine must have been called before this method.
   void adjustToUnmodifiedLine(const AnnotatedLine &Line) {
+    if (!Line.InPPDirective) {
       unsigned LevelIndent = Line.First->OriginalColumn;
       if (static_cast<int>(LevelIndent) - Offset >= 0)
         LevelIndent -= Offset;
       assert(Line.Level < IndentForLevel.size());
-    if ((!Line.First->is(tok::comment) || IndentForLevel[Line.Level] == -1) &&
-        !Line.InPPDirective) {
+      if (Line.First->isNot(tok::comment) || IndentForLevel[Line.Level] == -1)
         IndentForLevel[Line.Level] = LevelIndent;
     }
   }
 
 private:
   /// Get the offset of the line relatively to the level.
   ///
   /// For example, 'public:' labels in classes are offset by 1 or 2
   /// characters to the left from their level.
private:
   }
 
   /// Get the indent of \p Level from \p IndentForLevel.
   ///
   /// \p IndentForLevel must contain the indent for the level \c l
   /// at \p IndentForLevel[l], or a value < 0 if the indent for
   /// that level is unknown.
   unsigned getIndent(unsigned Level) const {
+    assert(Level < IndentForLevel.size());
     if (IndentForLevel[Level] != -1)
       return IndentForLevel[Level];
     if (Level == 0)
       return 0;
     return getIndent(Level - 1) + Style.IndentWidth;
   }
 
   const FormatStyle &Style;
   const AdditionalKeywords &Keywords;
   const unsigned AdditionalIndent;
 
-  /// The indent in characters for each level.
+  /// The indent in characters for each level. It remembers the indent of
+  /// previous lines (that are not PP directives) of equal or lower levels. This
+  /// is used to align formatted lines to the indent of previous non-formatted
+  /// lines. Think about the --lines parameter of clang-format.
   SmallVector<int> IndentForLevel;
 
   /// Offset of the current line relative to the indent level.
   ///
   /// For example, the 'public' keywords is often indented with a negative
   /// offset.
   int Offset = 0;