Revert "Fix nested namespaces in google-readability-nested-namespace-comments."

This reverts r315057. The revision introduces assertion failures:
assertion failed at llvm/tools/clang/include/clang/Basic/SourceManager.h:428 in
const clang::SrcMgr::ExpansionInfo &clang::SrcMgr::SLocEntry::getExpansion()
const: isExpansion() && "Not a macro expansion SLocEntry!"
Stack trace:
    __assert_fail
    clang::SrcMgr::SLocEntry::getExpansion()
    clang::SourceManager::getExpansionLocSlowCase()
    clang::SourceManager::getExpansionLoc()
    clang::Lexer::getRawToken()
    clang::tidy::readability::NamespaceCommentCheck::check()
    clang::ast_matchers::internal::(anonymous namespace)::MatchASTVisitor::MatchVisitor::visitMatch()
    clang::ast_matchers::internal::BoundNodesTreeBuilder::visitMatches()
    clang::ast_matchers::internal::(anonymous namespace)::MatchASTVisitor::matchWithFilter()
    clang::ast_matchers::internal::(anonymous namespace)::MatchASTVisitor::matchDispatch()
    clang::ast_matchers::internal::(anonymous namespace)::MatchASTVisitor::TraverseDecl()
    clang::RecursiveASTVisitor<>::TraverseDeclContextHelper()
    clang::RecursiveASTVisitor<>::TraverseDecl()
    clang::RecursiveASTVisitor<>::TraverseDeclContextHelper()
    clang::RecursiveASTVisitor<>::TraverseDecl()
    clang::RecursiveASTVisitor<>::TraverseDeclContextHelper()
    clang::RecursiveASTVisitor<>::TraverseDecl()
    clang::ast_matchers::MatchFinder::matchAST()
    clang::MultiplexConsumer::HandleTranslationUnit()
    clang::ParseAST()
    clang::FrontendAction::Execute()
    clang::CompilerInstance::ExecuteAction()
    clang::tooling::FrontendActionFactory::runInvocation()
    clang::tooling::ToolInvocation::runInvocation()
    clang::tooling::ToolInvocation::run()

Still working on an isolated test case.

llvm-svn: 315580