[clang-format] Move OriginalPrefix from base to subclass.

[krasimir]

Summary

OriginalPrefix is only needed for line comment sections. Moved from the base class to the child class.
No functional changes.

[djasper]

lg

lib/Format/BreakableToken.h

protected:
 
   // The original start column of the first line of text from this section.
   unsigned OriginalStartColumn;
 
   // Whether the first token of this section is the first token in its unwrapped
   // line.
   bool FirstInLine;
 
-  // In case of line comments, holds the original prefix, including trailing
-  // whitespace.
-  SmallVector<StringRef, 16> OriginalPrefix;
-
   // The prefix to use in front a line that has been reflown up.
   // For example, when reflowing the second line after the first here:
   // // comment 1
   // // comment 2
   // we expect:
   // // comment 1 comment 2
   // and not:
   // // comment 1comment 2
public:
   void updateNextToken(LineState& State) const override;
   bool mayReflow(unsigned LineIndex,
                  llvm::Regex &CommentPragmasRegex) const override;
 
 private:
   unsigned getContentStartColumn(unsigned LineIndex,
                                  unsigned TailOffset) const override;
 
+  // OriginalPrefix[i] contains the original prefix of line i, including
+  // trailing whitespace before the start of the content. The indentation
+  // preceding the prefix is not included.
+  // For example, if the line is:
+  // // content
+  // then the original prefix is "// ".
+  SmallVector<StringRef, 16> OriginalPrefix;
+
   // Prefix[i] contains the intended leading "//" with trailing spaces to
   // account for the indentation of content within the comment at line i after
   // formatting. It can be different than the original prefix when the original
   // line starts like this:
   // //content
   // Then the original prefix is "//", but the prefix is "// ".
   SmallVector<StringRef, 16> Prefix;