Add ir::{lookup_data_member, get_function_parameter}

Add ir::{lookup_data_member, get_function_parameter}

While looking at something else, I figured it's useful, for debugging
purposes, to be able to lookup a given data member of a union/class by
name, as well as a function parameter by index.

This patch adds both.

	* include/abg-ir.h (lookup_data_member, get_function_parameter):
	Declare new functions.
	* src/abg-ir.cc (lookup_data_member, get_function_parameter):
	Define them.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>