clang-format: fix prefix for doxygen comments after member

Summary:
Doxygen supports putting documentation blocks after member, by adding
an additional < marker in the comment block. This patch makes sure
this marker is used in lines which are introduced by breaking the
comment.

  int foo; ///< Some very long comment.

becomes:

  int foo; ///< Some very long
           ///< comment.

Contributed by @Typz!

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: djasper, klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D33282

llvm-svn: 303330