[clang-tidy] Move the missing namespace comment warnings to the closing brace

Summary:
The google-readability-namespace-comments/llvm-namespace-comment
warnings are quite confusing when they appear at the beginning of a long
namespace and the closing brace is not in sight.

For convenience added notes pointing to the start of the namespace.

Reviewers: klimek

Reviewed By: klimek

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D6251