[clang-tidy] Fix google-readability-namespace-comments handling of C++17 nested namespaces

Summary:
Fixes bug 34701

When we encounter a namespace find the location of the left bracket.
Then if the text between the name and the left bracket contains a ':'
then it's a C++17 nested namespace.

Reviewers: #clang-tools-extra, alexfh, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: curdeius, cfe-commits, krasimir, JonasToth, JDevlieghere, xazax.hun

Tags: #clang-tools-extra

Patch by Alexandru Octavian Buțiu!

Differential Revision: https://reviews.llvm.org/D38284

llvm-svn: 322274