[clang-format] Add a new flag FixNamespaceComments to FormatStyle

Summary:
This patch enables namespace end comments under a new flag FixNamespaceComments,
which is enabled for the LLVM and Google styles.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D30405

llvm-svn: 296632