[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