interface: cpp_generator::get_method_kind: handle constructors as well
interface: cpp_generator::get_method_kind: handle constructors as well

In the next commit, this function will be turned into a Method method
and for uniformity it will be convenient for this method
to support isl class constructors as well.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl