[clang] Emit error for invalid friend functions under [temp.friend]p9 (#78083)

Emits an error for friend FunctionDecls that either:

* are not templates and have a requires clause
* are templates, and have a constrained parameter that depends on a
template parameter from an enclosing template

and are not a definition.

For a non-template friend function with a requires clause, if the
function is not templated then the original error message indicating
that such a function is disallowed is shown instead, as the function
will still be rejected if a definition is added.