[clang-tidy] Minor fixes for the NamespaceCommentCheck.

  * Make SmallVector size enough for all groups.
  * Allow trailing period in the comment.
  * Fix "// anonymous namespace qqq".



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@219926 91177308-0d34-0410-b5e6-96231b3b80d8