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

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

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29716

llvm-svn: 294457