Add an option to ignore warnings for cppcoreguidelines
avoid-non-const-global-variables.
Understandably, the core guidelines discourage non const global
variables, even at the TU level (see
https://github.com/isocpp/CppCoreGuidelines/issues/2195). However,
having a small TU with an interface that uses a non const variable from
an anonymous namespace can be a valid choice.
This adds an option that disables the warning just for anonymous
namespaces, i.e. at the file level. The default is still to show a
warning, just as before.
7c50187b7d