[Bug c++/65047] [c++17] Add support for nested namespace defintions.

[andrea.azzarone at canonical dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65047

--- Comment #1 from Andrea Azzarone <andrea.azzarone at canonical dot com> ---
Created attachment 34745
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34745&action=edit
Proposed patch