Add some wording to the coding standards to say how to indent namespaces
(and to mention namespace ending comments). This is based on a quick
discussion on the developer mailing list where there was essentially no
objections to a simple and consistent rule. This should avoid future
debates about whether or not a namespace is "big enough" to indent. It
also matches clang-format's current behavior with LLVM source code which
hasn't really seen any opposition in code reviews that I spot checked.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199620 91177308-0d34-0410-b5e6-96231b3b80d8