[clang-tidy] Adjust location of namespace comment diagnostic

Summary:
If there is no comment, place it at the closing brace of a namespace
definition. Previously it was placed at the next character after the
closing brace.

The new position produces a better location for highlighting in clangd
and does not seem to make matters worse for clang-tidy.

Reviewers: alexfh, hokein

Reviewed By: alexfh, hokein

Subscribers: xazax.hun, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64861

llvm-svn: 366337