`unused` warnings can be triggered even though the template is referenced by another template

[ojhunt]

If you have a template in an anonymous namespace we will warn about it being unused even if it is referenced by another template, if the other template is not instantiated.

i.e compiling the following with -Wunused-template

namespace {
  template <class T> void foo(T*){}
}

template <class T> void bar(T* t) {
  foo(t);
}

produces
warning: unused function template 'foo' [-Wunused-template]

The only way to silence this is by using a [[maybe_unused]], which is a solution but is somewhat suboptimal.

[ojhunt]

Also applies to unused_function warning.

I suspect the fix is to record free functions, etc in template decls and see if they could resolve to an "unused" decl