[metadata] Implement MONO_CLASS_IS_INTERFACE(c) in terms of public API
Althought it was previously defined in a public API header, the macro was not
usable for the last several years since it was implemented in terms of
the MonoClass definition which was not visible in the public API.
There is a subtle semantic change here: mono_type_is_generic_parameter will
return FALSE if it's given a byref type parameter, the previous definition of
the macro would return TRUE. (E.g. if you had "class F<T>" then
IS_INTERFACE("T&") used to return TRUE but will now return FALSE. )