[clang-tidy] Replace unrecognized namespace ending comments.

Summary:
Replace unrecognized namespace ending comments. This will help in particular when a namespace ending comment is mistyped or doesn't fit the regexp for other reason, e.g.:

  namespace a {
  namespace b {
  namespace {
  } // anoynmous namespace
  } // b
  } // namesapce a

Reviewers: djasper

Reviewed By: djasper

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D8078

llvm-svn: 231369