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

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.

Differential Revision: https://reviews.llvm.org/D155094

clang/lib/Format/UnwrappedLineFormatter.cpp

@@ -88,14 +88,15 @@ class LevelIndentTracker {
   /// level to the same indent.
   /// Note that \c nextLine must have been called before this method.
   void adjustToUnmodifiedLine(const AnnotatedLine &Line) {
+    if (Line.InPPDirective)
+      return;
+    assert(Line.Level < IndentForLevel.size());
+    if (Line.First->is(tok::comment) && IndentForLevel[Line.Level] != -1)
+      return;
     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) {
-      IndentForLevel[Line.Level] = LevelIndent;
-    }
+    IndentForLevel[Line.Level] = LevelIndent;
   }
 
 private:
@@ -148,6 +149,7 @@ class LevelIndentTracker {
   /// 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)
@@ -159,7 +161,10 @@ class LevelIndentTracker {
   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.