Make var_diff and function_decl_diff extend decl_diff_base

Make var_diff and function_decl_diff extend decl_diff_base

	* include/abg-comparison.h (class var_diff, class
	function_decl_diff): Make these types inherit the decl_diff_base
	class.  This is like the fact that all kinds of diff inherit the
	type_diff_base class.
	* src/abg-comparison.cc (var_diff::var_diff)
	(function_decl_diff::function_decl_diff): Adjust to initialize the
	sub-object of decl_diff_base;
	(is_decl_diff): New predicate to know if a diff is about decls.
	(is_function_decl): New predicate to know if a diff is about
	function decls.
	(function_suppression::suppresses_diff): Adjust to use the new
	is_function_decl.

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